Fudgy Sourdough Brownies

Ultra fudgy brownies with complex chocolate flavor developed through 12-18 hours of cold fermentation. The overnight rest creates incredible depth while maintaining that rich, decadent texture everyone loves.

Course: Dessert

Cuisine: American

Keyword: chocolate brownies, fermented brownies, fudgy brownies, overnight brownies, sourdough brownies

Servings: 16

Author: Noelle Reed

Ingredients

170 g unsalted butter
140 g semi-sweet chocolate chopped or chips
200 g granulated sugar
60 g brown sugar
150 g eggs about 3 large
20 g egg yolk about 1 extra yolk
8 g vanilla extract roughly 2 tsp
60 g unsweetened cocoa powder
3 g espresso powder about 1 tsp, optional but highly recommended
85 g all-purpose flour
80 g sourdough starter 100% hydration, discard or active
4 g fine salt
120 g dark chocolate chips for folding in before fermentation
Flaky sea salt for topping

Instructions

Mix Wet Ingredients:

Melt chocolate + butter: In a saucepan or bowl over gentle heat, melt the butter and chopped chocolate together until smooth. Set aside to cool slightly.
Add sugars, eggs, vanilla, and starter: Whisk in both sugars while the mixture is still warm, then beat in the eggs and extra yolk one at a time. Stir in the vanilla extract and sourdough starter until everything is glossy and cohesive.

Mix dry ingredients:

In a separate bowl, combine cocoa powder, espresso powder, flour, and salt. Fold this gently into the wet mixture until just combined—no overmixing.
Fold in the chocolate chips.

Ferment overnight:

Cover the bowl and refrigerate for 12–18 hours. This slow ferment deepens flavor and helps develop a fudgy, complex crumb.

Next day, prepare to bake:

Let the batter sit at room temperature for 60-75 minutes to loosen up properly.
Pour into a parchment-lined pan (see notes for sizes) and smooth the top.
Sprinkle with flaky sea salt.
Bake at 340°F (170°C): 8×8” pan: 32-38 minutes, 9×9” pan: 28-32 minutes, 13×9” pan: 22-26 minutes. You’re looking for set edges and a center that’s just barely jiggly—don’t overbake! The top should look slightly underbaked when you remove them.
Cool completely: Let cool completely in the pan before slicing to preserve the ultra-fudgy center. For cleanest cuts, chill for 1 hour before slicing.

Notes

Don’t skip the overnight fermentation, it’s what makes these special

Pan options:

8x8” for extra thick brownies (32-38 min)

9x9” for thick, decadent squares (28-32 min)

13x9” for thinner, easier-to-serve brownies (22-26 min)

Bring batter to room temperature:

After the overnight rest, let sit at room temp for 60-75 minutes before baking. This is essential for even baking.

Don’t overbake (unless you like those edges extra crispy):

Brownies should look slightly underbaked when removed. Center should be just barely jiggly with set edges. They continue cooking as they cool.

Cool completely:

Let cool completely in the pan before slicing. For cleanest cuts, chill for 1 hour before cutting.

Use quality chocolate:

Your chocolate is a main ingredient here. Use chocolate you’d be happy eating on its own.

Measure by weight:

Use a kitchen scale and measure in grams for the most accurate results.

Flaky salt is essential:

Don’t skip the flaky sea salt on top. It balances the sweetness and brings out the chocolate flavor.

Storage:

Store in an airtight container at room temperature for up to 4 days, refrigerate for up to a week, or freeze for up to 3 months.

No sourdough starter?

Replace the 80g starter with 40g all-purpose flour and 40g milk.

Mix and bake immediately (no overnight rest). No baking powder or baking soda needed.