If you have made my Sourdough S’mores Cookies, you already know I am obsessed with them. But sometimes you want all of that gooey marshmallow, melted chocolate, and fermented graham cracker crumble magic without the individual cookie assembly. These sourdough s’mores cookie bars are exactly that. One pan, one bake, slice and serve.
Same cold fermented cookie dough. Same fermented graham cracker crumble. Same frozen marshmallows and chopped chocolate. Just built into a 13×9 pan using the upside down method, flipped onto a cutting board, and sliced into bars with a marbled top that people will not stop photographing.
The upside down assembly does all the heavy lifting here. Everything goes into the pan in reverse so when you flip, you get that stunning marbled top with gooey marshmallow, pools of dark chocolate, and golden graham cracker crumble baked right in. No campfire required.
Table of Contents
- Sourdough Recipe
- Why You Will Love These Bars
- Ingredients You Need
- How the Upside Down Bar Method Works
- Fermentation: Quick or Overnight
- Making These Bars
- Pro Tips for Success
- Can These Be Made Without Sourdough Starter? (Printable Recipe)
- Substitutions and Variations
- Storage and Serving
- Frequently Asked Questions
- Never Miss a Recipe
Recipe
Sourdough S’mores Cookie Bars
Equipment
- 13×9 inch baking pan 1/4 sheet pan
- Parchment paper
- Stand mixer or hand mixer
- Food processor or pastry cutter
- Rolling Pin
- plastic wrap
Ingredients
Sourdough Cookie Dough
- 120 g active sourdough starter or discard 1/2 cup
- 113 g unsalted butter softened (1/2 cup)
- 80 g powdered sugar 2/3 cup
- 67 g brown sugar 1/3 cup packed
- 50 g egg 1 large egg
- 1 teaspoon vanilla extract
- 2.5 g almond extract 1/2 teaspoon, optional
- 270 g all purpose flour 2 1/4 cups
- 2 g baking powder 1/2 teaspoon
- 1.5 g salt 1/4 teaspoon
- 0.5 g cream of tartar 1/8 teaspoon
Fermented Graham Cracker Crumble
- 60 g graham crackers broken into pieces and crumbs (about 8 full cracker sheets)
- 60 g all purpose flour 1/2 cup
- 25 g active sourdough starter or discard 2 Tablespoons
- 35 g brown sugar 3 tablespoons packed
- 20 g powdered sugar 3 tablespoons
- 45 g cold unsalted butter cubed (3 tablespoons)
- 4 g salt 3/4 teaspoon
- 3 g vanilla extract 3/4 teaspoon
S’mores Layer
- 60 g mini marshmallows frozen (1 1/4 cups)
- 80 g chocolate bar roughly chopped into chunks (about 2.8 oz)
- 1 teaspoon cornstarch
Instructions
Make the Dough and Crumble
- In a large bowl or stand mixer, cream together softened butter, powdered sugar, and brown sugar until light and fluffy.
- Add the egg, vanilla extract, and almond extract if using. Mix until well combined.
- Add the sourdough starter and mix until fully incorporated.
- In a separate bowl, whisk together flour, baking powder, salt, and cream of tartar.
- Gradually add the dry ingredients to the wet ingredients and mix until just combined. Do not overmix.
- Wrap the dough tightly in plastic wrap and refrigerate.
For the crumble
- Food Processor, add flour, brown sugar, powdered sugar, and salt to a food processor and pulse a few times to combine.
- Add the graham cracker pieces and pulse until you have a mix of larger chunks and fine crumbs with real texture.
- Add the cold cubed butter and pulse until incorporated.
- Add the sourdough starter and vanilla extract and pulse until the mixture reaches a brown sugar texture. It should hold together when squeezed and crumble apart easily when broken.
- Transfer the crumble to a container, cover, and refrigerate alongside the dough for a minimum of 2 hours and up to 48 hours.
- To Do by HandPlace graham crackers in a zip-top bag and crush with a rolling pin until you have a mix of chunks and fine crumbs. Add crushed crackers, flour, brown sugar, powdered sugar, and salt to a bowl and whisk together. Cut in the cold cubed butter with a pastry cutter or your fingertips until the mixture resembles coarse crumbs. Add vanilla and sourdough starter and work until the mixture holds together when squeezed but crumbles apart easily. Work quickly to keep the butter cold. Transfer the crumble to a container, cover, and refrigerate alongside the dough for a minimum of 2 hours and up to 48 hours.
Assemble and Bake
- Toss the mini marshmallows with the cornstarch until lightly coated. Place in the freezer for at least 30 minutes before assembly. Pull out the dough and let rest for 15 minutes.
- Preheat oven to 350 degrees F (175 degrees C).
- Line a 13×9 inch baking pan with parchment paper, leaving overhang on all four sides.
- Optional, Break a few graham cracker squares from the crumble mixture into irregular pieces and scatter them loosely across the parchment. Leave gaps between the pieces.
- Scatter the fermented crumble over and around the graham cracker pieces, spreading it across the entire pan. Push the crumble firmly all the way to the edges and corners of the pan to seal the border.
- Scatter the chopped chocolate chunks evenly over the crumble layer.
- Add the frozen cornstarch dusted marshmallows in an even layer over the chocolate.
- Push the chocolate and marshmallows around so the break through to the bottom of the pan to have that beautiful marbled top.
- Trace the bottom of your 13×9 pan onto a separate sheet of parchment paper. Use that outline as a guide and roll the cold fermented cookie dough to match the shape, approximately 3/8 inch thick.
- Carefully transfer the dough slab into the pan over the s’mores layers. Press it down gently to cover everything completely, pressing into the corners and edges with lightly floured fingers.
- Bake for 20 to 30 minutes until the edges are golden and the top of the cookie slab is fully set. The marshmallows will have puffed up the dough. The sweet spot is usually around 25 minutes but every oven is different.
- Remove from oven and let cool in the pan for a full 45 minutes before flipping.
- Use a piece of parchment and lay it over top of the dough top. Place a large cutting board over the top and flip in one confident motion. Peel the parchment away carefully.
- Let the bars continue cooling at room temperature until the chocolate is fully set before slicing.
- Use a sharp knife and make clean decisive cuts. Wipe the blade between cuts for the cleanest edges.
Notes
Why You Will Love These Bars
The individual s’mores cookies are stunning and I will never stop making them. But the bar format gives you something the individual cookies cannot: a dramatic cross section in every single slice. When you cut into these sourdough s’mores cookie bars, every piece reveals layers of golden cookie, gooey marshmallow, pockets of melted chocolate, and that toasty fermented crumble. It is the kind of thing that makes people stop scrolling.
It is also just easier. One batch of dough, one pan, one bake. No cutting circles, no individual assembly, no managing crumble piles per cookie. Just press the dough slab over the s’mores layers, bake, flip, and slice.
The crumble layer in these bars is built two ways. First, broken graham cracker pieces go directly onto the parchment. Then the fermented crumble mixture scatters over and around those pieces, filling in the gaps. When the bars flip out of the pan, you get a marbled top with visible graham cracker shards, crumble, marshmallow, and chocolate all baked together. That texture contrast is everything.
Ingredients You Need
These sourdough s’mores cookie bars use the same three component structure as the original cookies.
For the sourdough cookie base: active sourdough starter or discard, unsalted butter, powdered sugar, brown sugar, one large egg, vanilla extract, almond extract (optional but worth it), all purpose flour, baking powder, salt, and cream of tartar.
For the fermented graham cracker crumble: graham crackers (regular or gluten free, both work perfectly), all purpose flour, sourdough starter or discard, brown sugar, powdered sugar, cold cubed butter, salt, and vanilla extract.
For the s’mores layer: mini marshmallows, a chocolate bar chopped into chunks, and cornstarch for dusting the marshmallows before freezing.
A few ingredient notes worth mentioning:
The sourdough starter goes into both the cookie dough and the crumble. You can use active peaked starter or discard straight from the fridge. Both work. The fermentation happens in the refrigerator either way, so use whichever you have on hand.
For the chocolate, use whatever bar you would actually eat on its own. Chopped chocolate bars melt into gorgeous pools that chips never quite achieve. Milk chocolate gives you the most classic s’mores flavor. Dark chocolate is a little less sweet and more complex. White chocolate paired with the graham cracker crumble is a really fun direction if you want something different.
Gluten free graham crackers work just as well as regular in both the crumble and the broken piece layer. Zero difference in the final result.
How the Upside Down Bar Method Works
If you have made the Sourdough S’mores Cookies before, this is the same logic scaled to a full pan. If the upside down method is new to you, here is what is happening.
Traditional bars are built bottom to top: base layer pressed into the pan, fillings added on top. The problem with gooey fillings like marshmallow and chocolate is that they can stick to the pan, seep through the base, or create a mess when you try to lift the bars out. The upside down method flips that entirely.
Everything is built in reverse onto a parchment lined pan. The crumble and graham cracker layer go down first and become what will eventually be the top of your finished bars. The s’mores fillings pile on next. The cookie dough gets pressed on top as a solid slab, sealing everything underneath. When it bakes, the dough bakes from the top down, the crumble gets golden and crisp against the parchment, and the marshmallows and chocolate stay completely contained inside.
Once the bars cool and you flip them, the crumble is the gorgeous marbled top. The parchment peels away cleanly. You slice and serve.
This is the same method used in the Berry Crumb Sourdough Cookie Squares, and I am building it out across the entire upside down cookie series. More bar versions are coming.
Fermentation: Quick or Overnight
These sourdough s’mores cookie bars are flexible on fermentation time, which makes them easy to fit into a real schedule.
The minimum is about two hours. Once the dough and crumble are made and chilled, you can assemble and bake right away. The texture is great and the bake works perfectly.
For more depth of flavor, ferment overnight. The sweet spot is 12 to 24 hours in the refrigerator. Both the cookie dough and the crumble ferment at the same time, so you make everything on day one and bake on day two. The cold fermentation develops a complexity that is subtle but real. It is the thing that makes people ask what is different without being able to name it.
The dough and crumble keep in the refrigerator for up to 48 hours, so if life gets in the way between day one and day two, you have some extra flexibility built in.
Making These Bars
The full ingredients and step by step instructions are in the recipe card below. Here is a look at the process with some tips woven in so you know what to expect before you start.
Day 1 is all about making the dough and crumble and getting them into the refrigerator. The cookie dough comes together quickly in a stand mixer. The crumble is easiest in a food processor but a pastry cutter works just fine too. You are aiming for that brown sugar texture where the crumble holds together when squeezed but breaks apart easily. Both go into the fridge wrapped tightly.
Day 2 starts with the marshmallows. Toss them in cornstarch and freeze for at least 30 minutes before you do anything else. This is not optional. The cornstarch creates a barrier that slows melting, and the freeze gives the cookie structure time to set before the marshmallows fully soften. Frozen marshmallows coated in cornstarch stay mostly intact long enough for the dough to bake around them properly.
Once the marshmallows are freezing, build your layers. Graham cracker pieces go down first in a loose, rustic scatter directly on the parchment. Not edge to edge, just loosely placed with gaps. The fermented crumble goes over and around those pieces, and this is where you want to be intentional: push the crumble all the way to the edges of the pan. That border seal keeps the marshmallows from bubbling out the sides during the bake. Chocolate chunks go next, then the frozen marshmallows.
For the dough slab, here is a tip that makes this so much easier: trace the bottom of your 13×9 pan onto a sheet of parchment paper and use that outline as your guide when rolling. Roll the cold fermented dough to match the shape, about 3/8 inch thick, then carefully transfer it into the pan over the s’mores layers and press it down to cover everything completely.
After baking, let the bars cool in the pan for a full 45 minutes before flipping. The structure needs that time to set. Once flipped, let the chocolate continue setting at room temperature before slicing. A sharp knife and decisive cuts give you the cleanest bars.
Pro Tips for Success
Freeze the marshmallows, no exceptions. Room temperature marshmallows melt out and pool on the pan before the cookie sets. Frozen marshmallows coated in cornstarch stay mostly intact long enough for everything to bake around them properly.
Push the crumble all the way to the edges. This seals the perimeter and keeps the marshmallows from bubbling out the sides during the bake. Every bar will also have crumble all the way to the edge when you cut.
Trace the pan for the dough slab. Rolling the dough to match a traced outline makes the transfer so much cleaner and gives you a more even thickness across the whole slab than eyeballing it.
Leave gaps in the crumble and graham cracker layer. Those gaps are where the marshmallow and chocolate peek through after flipping and create the marbled effect. A solid packed crumble layer blocks that.
Wait the full 45 minutes before flipping. The bars need that time to set up structurally. Flipping too early means the slab cracks or falls apart when you lift it.
Cool completely before slicing. The chocolate needs to set fully. Slicing while everything is still warm gives you a messy cut instead of a clean bar. Unless you are into that, in which case no judgment.
Can These Be Made Without Sourdough Starter?
Yes, absolutely. You do not need a sourdough starter to make these bars. The upside down method, the layers, the marbled top, the gooey marshmallow and chocolate all work exactly the same way without it. Here is everything you need to know to make the swap confidently.
Why sourdough starter is in this recipe in the first place
The starter serves two purposes here. First, it adds moisture and a slight acidity that tenderizes the cookie dough and crumble. Second, when given time to ferment in the refrigerator, it develops a subtle depth of flavor that makes people reach for a second bar without being able to explain why. It does not make these taste like bread. It just makes them taste more interesting.
Without the starter, you get a slightly more straightforward cookie bar that is still completely delicious, especially with the s’mores layers doing so much of the flavor work.
S’mores Cookie Bars (No Sourdough Starter)
Equipment
- 13×9 inch baking pan
- Parchment paper
- Stand mixer or hand mixer
- Food processor or pastry cutter
- Rolling Pin
- plastic wrap
Ingredients
Cookie Dough
- 330 g all purpose flour 2 3/4 cups
- 60 g whole milk 1/4 cup
- 113 g unsalted butter softened (1/2 cup)
- 80 g powdered sugar 2/3 cup
- 67 g brown sugar 1/3 cup packed
- 50 g egg 1 large egg
- 1 teaspoon vanilla extract
- 2.5 g almond extract 1/2 teaspoon, optional
- 2 g baking powder 1/2 teaspoon
- 1.5 g salt 1/4 teaspoon
- 0.5 g cream of tartar 1/8 teaspoon
Graham Cracker Crumble
- 60 g graham crackers broken into pieces and crumbs (about 8 full cracker sheets)
- 68 g all purpose flour 1/2 cup plus 1 tablespoon
- 35 g brown sugar 3 tablespoons packed
- 20 g powdered sugar 3 tablespoons
- 45 g cold unsalted butter cubed (3 tablespoons)
- 4 g salt 3/4 teaspoon
- 3 g vanilla extract 3/4 teaspoon
- 12 g whole milk 1 tablespoon
S’mores Layer
- 60 g mini marshmallows 1 1/4 cups
- 80 g chocolate bar roughly chopped into chunks (about 2.8 oz)
- 1 teaspoon cornstarch
Instructions
Make the Dough and Crumble
- In a large bowl or stand mixer, cream together softened butter, powdered sugar, and brown sugar until light and fluffy.
- Add the egg, vanilla extract, and almond extract if using. Mix until well combined.
- Add the milk and mix to combine.
- In a separate bowl, whisk together the flour, baking powder, salt, and cream of tartar.
- Gradually add the dry ingredients to the wet ingredients and mix until just combined. Do not overmix.
- Wrap the dough tightly in plastic wrap and refrigerate for at least 2 hours.
For the crumble
- For the food processor (my prefered method), add flour, brown sugar, powdered sugar, and salt to a food processor and pulse a few times to combine.
- Add the graham cracker pieces and pulse until you have a mix of larger chunks and fine crumbs with real texture.
- Add the cold cubed butter and pulse until incorporated.
- Drizzle in the vanilla and milk and continue pulsing until the mixture reaches a brown sugar texture. It should hold together when squeezed and crumble apart easily when broken.
- Transfer the crumble to a container, cover, and refrigerate alongside the dough for at least 2 hours.
- To Hand Cut the Crumble, Place graham crackers in a zip-top bag and crush with a rolling pin until you have a mix of chunks and fine crumbs. Add crushed crackers, flour, brown sugar, powdered sugar, and salt to a bowl and whisk together. Cut in the cold cubed butter with a pastry cutter or your fingertips until the mixture resembles coarse crumbs. Add vanilla and milk and work until the mixture holds together when squeezed but crumbles apart easily. Work quickly to keep the butter cold. Transfer the crumble to a container, cover, and refrigerate alongside the dough for at least 2 hours.
Assemble and Bake
- Toss the mini marshmallows with the cornstarch until lightly coated. Place in the freezer for at least 30 minutes before assembly. Pull your dough out and let rest for 15 minutes before rolling. If it would get too warm, place back in the fridge for 15 minutes.
- Preheat oven to 350 degrees F (175 degrees C).
- Line a 13×9 inch baking pan with parchment paper, leaving overhang on all four sides.
- Break a few graham cracker squares from the crumble mixture into irregular pieces and scatter them loosely across the parchment. Leave gaps between the pieces.
- Scatter the crumble over and around the graham cracker pieces, spreading it across the entire pan. Push the crumble firmly all the way to the edges and corners of the pan to seal the border.
- Scatter the chopped chocolate chunks evenly over the crumble layer.
- Add the frozen cornstarch dusted marshmallows in an even layer over the chocolate.
- Trace the bottom of your 13×9 pan onto a separate sheet of parchment paper. Use that outline as a guide and roll the cold cookie dough to match the shape, approximately 3/8 inch thick.
- Carefully transfer the dough slab into the pan over the s’mores layers. Press it down gently to cover everything completely, pressing into the corners and edges with lightly floured fingers.
- Bake for 20 to 30 minutes until the edges are golden and the top of the cookie slab is fully set. The marshmallows will have puffed up through the dough. The sweet spot is usually around 25 minutes but every oven is different.
- Remove from oven and let cool in the pan for a full 45 minutes before flipping.
- Use the parchment overhang to lift the entire slab out of the pan. Place a large cutting board over the top and flip in one confident motion. Peel the parchment away carefully.
- Let the bars continue cooling at room temperature until the chocolate is fully set before slicing.
- Use a sharp knife and make clean decisive cuts. Wipe the blade between cuts for the cleanest edges.
Notes
What you will notice in the finished bar
The texture will be very similar. The crumble will still crisp up beautifully. The marshmallow will still be gooey. The chocolate will still melt into pools. The marbled top will still look stunning. The one thing you will not get is that subtle complexity that comes from fermentation, but the s’mores layers are bold enough to carry the whole bar on their own.
Substitutions and Variations
Graham crackers: Regular or gluten free work equally well in both the crumble and the broken piece layer. No adjustments needed either way.
Chocolate: Use whatever bar you would actually eat on its own. Milk chocolate is the most classic s’mores flavor. Dark chocolate is less sweet and more complex. White chocolate paired with the graham cracker crumble is a genuinely fun variation. Avoid chips, which hold their shape instead of melting into pools the way a chopped bar does.
Large marshmallows: If you only have regular sized marshmallows, cut them into quarters before the cornstarch and freezing step.
Almond extract: Listed as optional in the cookie dough, but it is one of those additions that makes people ask what that subtle flavor is without being able to identify it.
Discard version: This is a great use for sourdough discard that has been in the fridge for a few days. The cold discard adds even more complexity to the fermented flavor.
Storage and Serving
These sourdough s’mores cookie bars are best the day they are baked when the crumble is at its crispest and the marshmallow is at its gooiest.
Store covered at room temperature for up to 2 days. Refrigerate after that for up to one week. A quick 10 second warm in the microwave brings the marshmallow right back to life.
To make ahead, the cookie dough and crumble both keep in the refrigerator for up to 48 hours. Assemble and bake on the day you plan to serve them for the best results.
Frequently Asked Questions
Can I make these without sourdough starter?
Yes. See the full breakdown in the section above for printable ingredient recipe. The bars bake and slice the same way.
Do I have to ferment overnight?
No. Once the dough and crumble are chilled, about two hours minimum, you can assemble and bake. The overnight fermentation adds more flavor complexity but the quick version is still delicious.
Why are my marshmallows melting out?
Two things: make sure they are frozen for the full 30 minutes before assembly and make sure they are thoroughly coated in cornstarch. Also push the crumble firmly to the edges of the pan so there is no gap for the marshmallow to escape through.
Can I use a quarter sheet pan instead of a 13×9?
Yes. A quarter sheet pan works well and gives slightly better heat circulation for crisping the crumble. The bake time stays the same.
Why do I need to wait 45 minutes before flipping?
The bars need time to set structurally after coming out of the oven. The chocolate and marshmallow are still molten inside and the cookie needs to firm up so it holds together when you flip and lift.
Can I use different chocolate?
Absolutely. Any chocolate bar you enjoy eating works here. Chopped bars melt more dramatically than chips and create those gooey pockets that feel most like a real s’more.
Do these taste like sourdough bread?
Not at all. The fermentation develops subtle depth and complexity without adding any bread like quality. These taste like a gourmet campfire treat, nothing more.
If you love the upside down cookie series, check out the full lineup: Sourdough S’mores Cookies and Berry Crumb Sourdough Cookie Squares. More upside down bar versions are on the way.
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