Fudgy sourdough brownies with a peanut butter swirl — simple, one-bowl treats that deliver rich chocolate flavor and a creamy peanut butter ribbon in every bite. If you love dense, chocolatey bars, these sourdough peanut butter brownies are an easy lunch-box favorite. The batter comes together quickly and bakes into perfectly fudgy squares.
Why You’ll Love Sourdough Peanut Butter Brownies
- Sweet treat: The classic pairing of chocolate and peanut butter is irresistible in these fudgy bars.
- One bowl: Minimal cleanup — everything mixes in a single bowl for an easy bake.
- Quick and easy: The recipe is straightforward and ready in about 40 minutes, perfect for busy days.
Important Ingredients
- Sourdough discard — Use 100% hydration discard for best texture. Fresh, active starter works too, but older refrigerated discard will yield a stronger sour flavor.
- Chocolate — Good-quality chocolate or semi-sweet/bittersweet chips work well. The recipe uses about 3 oz (85 g) of chopped dark chocolate, melted.
- Peanut butter — A creamy, no-stir peanut butter gives the best texture for swirling.
- Unsalted butter — Baking with unsalted butter lets you control the salt level in the brownies.
- Cocoa powder — A dark or dutch-processed cocoa brings extra depth of chocolate flavor.
- Sugars — A mix of granulated and brown sugar balances sweetness and chewiness.
- Eggs — Eggs beaten with the sugars provide structure and a light crumb.
- Flour — All-purpose flour keeps the brownies fudgy when used in a small amount.
- Salt and vanilla — Both enhance and round out the chocolate-peanut butter flavor.
How to Make Sourdough Peanut Butter Brownies
Mix the batter
In a large bowl whisk together melted warm butter, brown sugar and granulated sugar until combined. Melt the chopped chocolate (or chips) in short bursts in the microwave, stirring every 20–30 seconds to prevent seizing, then stir the melted chocolate into the butter and sugar mixture.
Add the eggs one at a time, whisking after each addition until fully incorporated. Stir in the sourdough discard and vanilla until smooth.
Sift or sprinkle the cocoa powder, flour and salt over the wet ingredients and fold with a spatula until you have a thick, smooth batter. Reserve the peanut butter for the next step so you can swirl it on top.
Spread batter in pan
Line an 8 by 8-inch metal baking pan with parchment paper. Spread the brownie batter evenly in the pan. Spoon about nine small dollops (roughly 85 g / 1/3 cup total) of creamy peanut butter across the surface. Use a knife to swirl the peanut butter through the batter so it forms a marbled pattern.
Bake
Bake at 350ºF (175ºC) for about 35–40 minutes, until the edges are set and the center still has moist crumbs on a toothpick. For reliable results, the center should reach about 190ºF (88ºC) for fudgy brownies. Avoid over-baking — they will continue to set as they cool.
Allow the brownies to cool completely in the pan before slicing so they set up cleanly. Then cut into squares and enjoy.
Amy’s Recipe Tip
Add ingredients in the order listed. Whisking warm butter with the sugars helps them dissolve, and adding eggs one at a time gives the brownies their structure. There’s no chemical leavening in this recipe, so following the steps helps ensure a fudgy texture.
Substitutions
- Sourdough starter: Active starter can replace discard; a younger discard yields less sour flavor.
- Chocolate chips: Semi-sweet or dark chocolate chips can be used instead of chopped dark chocolate.
- Peanut butter: Any creamy, non-stir nut butter works, or omit it altogether if preferred.
- Cocoa powder: If you don’t have dutch-processed cocoa, regular cocoa will still produce good flavor.
How to Store Leftovers
Store brownies at room temperature for up to 24 hours. For longer storage, slice and freeze individual pieces for 2–3 months. Thaw at room temperature when ready to enjoy.
If you liked this, you’ll also like…
- Fudgy Sourdough Brownies
- Peanut Butter Cookies with Sourdough Discard
- Chocolate Cobbler with Sourdough Discard
- Chocolate Sourdough Zucchini Cake
- Sourdough Texas Sheet Cake
Frequently Asked Questions
A metal pan bakes more evenly, so it’s preferred. Glass retains heat longer and can lead to slightly more baked, drier edges if you don’t reduce baking time or temperature.
Yes — about 100 g of chopped nuts folded in after the batter is mixed adds a nice crunch.
They were likely over-baked. Aim for an internal temperature around 190ºF for fudgy brownies and remove them when a toothpick shows moist crumbs. They will continue to set as they cool.
Sourdough Peanut Butter Brownies
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Equipment
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8 by 8 pan
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Parchment paper
Ingredients
- 113 grams unsalted butter, melted and warm, 1/2 cup
- 140 grams brown sugar, 2/3 cup, packed
- 140 grams white sugar, 2/3 cup
- 85 grams chopped dark chocolate, melted, 3 oz
- 2 large eggs, 100 grams
- 100 grams sourdough discard, scant 1/2 cup
- 4 grams vanilla extract, 1 teaspoon
- 55 grams cocoa powder, dark or dutched, 2/3 cup
- 30 grams all-purpose flour, scant 1/4 cup
- 6 grams salt, about 1 teaspoon
- 85 grams creamy peanut butter, about 1/3 cup
Instructions
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Preheat oven to 350ºF. Line an 8 by 8-inch metal baking pan with parchment paper.
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In a large bowl whisk melted butter with brown and white sugar. Melt the chocolate in short bursts, stirring often to prevent seizing, then add it to the sugar-butter mixture and whisk until combined.
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Add eggs one at a time, whisking until fully incorporated. Stir in sourdough discard and vanilla until smooth.
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Fold in cocoa powder, flour and salt until you have a thick, even batter.
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Spread the batter in the prepared pan. Spoon small dollops of creamy peanut butter on top and use a knife to swirl it into the batter.
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Bake 35–40 minutes until edges are set and a toothpick in the center has moist crumbs. Aim for an internal temperature near 190ºF for fudgy brownies. Remove and allow to cool in the pan.
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Cool completely, slice into squares and serve. Enjoy!
Notes
Baking: The longer you bake, the more cake-like and dry the brownies become. For fudgy results, remove at about 190ºF; for a cakier texture, bake to a higher internal temperature.
Peanut butter: A no-stir creamy peanut butter performs best; natural peanut butters can separate and change the texture.
Nutrition
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Carbohydrates: 26g
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Protein: 3g
Nutrition information is automatically calculated and should be used as an approximation.
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