Go Back
sourdough smores cookie bars stacked showing gooey marshmallow and chocolate cross section layers on white surface
Print Recipe

Sourdough S'mores Cookie Bars

Cold fermented sourdough cookie bars with a fermented graham cracker crumble, gooey mini marshmallows, and melted chocolate chunks, built upside down in one pan and flipped for a stunning marbled top.
Course: Dessert
Cuisine: American
Keyword: fermented cookie bars, smores bars, sourdough cookie bars, sourdough discard dessert, sourdough smores cookie bars, upside down cookie bars
Calories: 365kcal

Equipment

  • 13x9 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.
    four panel collage showing how to make fermented graham cracker crumble for sourdough smores cookie bars in a food processor
  • 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 Hand
    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 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 13x9 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.
    six step collage showing how to build sourdough smores cookie bars with graham cracker pieces crumble chocolate marshmallows and cookie dough
  • Trace the bottom of your 13x9 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

Fermentation Timing
The minimum chill time is 2 hours. The sweet spot for flavor is 12 to 24 hours in the refrigerator. The dough and crumble keep for up to 48 hours, so you have flexibility if life gets in the way.
Tips
Do not skip freezing the marshmallows. Room temperature marshmallows melt out before the cookie sets. Frozen marshmallows coated in cornstarch stay mostly intact long enough for the dough to bake around them.
Push the crumble firmly to all four edges of the pan. This seals the border and keeps the marshmallows from bubbling out the sides during the bake.
Tracing the pan on parchment before rolling the dough makes the transfer much cleaner and gives you a more even slab.
Storage
Best the day they are baked. 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 back to life.
No Starter? No Problem
A full no starter version of this recipe with complete ingredient substitutions and instructions is in the recipe card below.
QR Code linking back to recipe