Sourdough Brownies Peanut Swirl

My sourdough starter had been neglected for weeks, sitting in the back of my fridge like a science experiment gone rogue. Instead of tossing it out, I wondered what would happen if I baked with it, and these brownies were born—fudgy, impossibly moist, with that subtle tang that makes you pause mid-bite. Now I almost look forward to having discard on hand because it means an excuse to make these.

I brought these to a neighborhood potluck last spring, and someone actually asked for the recipe while still chewing their first bite. What got me wasn't just the compliment—it was watching someone's whole face relax when they tasted the peanut butter and chocolate together, warm from my hands through their kitchen. That's when it clicked that using my sourdough discard wasn't just practical; it was kind of magic.

Ingredients

• Unsalted butter (115 g): The foundation for chocolate tempering—unsalted lets the chocolate sing without competing flavors.
• Semisweet chocolate chips or chopped chocolate (170 g): Quality matters here because there's nowhere for mediocre chocolate to hide, so pick something you'd actually eat on its own.
• Sourdough discard (120 g, unfed): This is the secret weapon that keeps these brownies tender and moist for days, plus it adds a whisper of complexity you can't quite put your finger on.
• Granulated sugar (200 g): The brownie's backbone for structure and that crucial shiny, crackly top.
• Large eggs (2): They bind everything while the yolks add richness that makes people think there's more butter than there actually is.
• Vanilla extract (1 tsp): A small amount that deepens the chocolate flavor without announcing itself.
• All-purpose flour (65 g): Keep it minimal—this is what separates fudgy brownies from cake-like ones.
• Unsweetened cocoa powder (30 g): The non-negotiable intensifier that punches up the chocolate flavor.
• Fine sea salt (1/2 tsp): Never skip this—it's the tiny thing that makes people say the chocolate tastes better.
• Creamy peanut butter (80 g): Use something smooth and spreadable rather than natural because you need it to swirl without fighting the batter.
• Powdered sugar (1 tbsp, optional): Makes the peanut butter swirl sweeter and slightly less thick, but honestly it's optional unless you're someone who loves that contrast.

Instructions

Get Your Oven Ready:

Preheat to 175°C (350°F) and line your 20x20 cm square pan with parchment paper—this matters because you want those brownies to come out clean, not stuck to the edges.

Melt the Chocolate and Butter:

Combine them in a microwave-safe bowl, working in 30-second bursts and stirring between each one so you don't end up with a seized, grainy mess. Once it's smooth and glossy, let it cool for a few minutes so it doesn't scramble your eggs.

Build Your Wet Base:

Whisk together the sourdough discard, sugar, eggs, and vanilla until they're completely combined and the mixture looks pale and slightly thickened. This is your time to really break down the sugar so it dissolves properly.

Bring It Together:

Pour the cooled chocolate into the egg mixture and whisk until you have a smooth, silky batter with no streaks—this only takes about a minute if your chocolate has cooled enough.

Fold in Your Dry Ingredients:

Sift the flour, cocoa powder, and salt directly into the batter, then use a spatula to fold gently until just combined. The moment you stop seeing dry flour, stop mixing—overmixing develops gluten and you'll end up with a cakey texture instead of fudgy.

Pour and Smooth:

Spread the batter evenly into your prepared pan, using the spatula to create a smooth, level surface.

Swirl in the Peanut Butter:

Mix the peanut butter and powdered sugar together in a small bowl until it's spreadable, then drop spoonfuls across the brownie batter in a loose grid. Use a skewer or thin knife to drag through the peanut butter and batter in alternating directions, creating those messy, organic lines that look homemade and somehow taste better because of it.

Bake Until Just Right:

Bake for 28–32 minutes—the center should still jiggle slightly when you gently shake the pan, and a toothpick should come out with fudgy crumbs, not wet batter. This is the difference between brownies that feel luxurious and ones that feel heavy.

Cool Completely:

Let them sit undisturbed in the pan until they've cooled completely, which takes longer than you think but is absolutely necessary so they hold together when you slice them.

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There's something quietly satisfying about watching someone take that first bite and immediately reach for another piece, no hesitation. It's the feeling that you've solved a small problem—turning something you were about to throw away into something that makes people happy.

Why Sourdough Discard Works Here

Most brownie recipes rely on enough fat and sugar alone to create that fudgy moisture, but sourdough discard adds something different—it contains fermented starches and a touch of acidity that keeps brownies moist and tender longer. The starter also brings subtle depth that makes chocolate taste more chocolate-y, almost like you've added espresso powder but more subtle. I've made these same brownies without the discard, and they're fine, but they dry out after two days whereas these stay soft for almost a week.

The Peanut Butter Swirl Technique

The key to a beautiful swirl is not overthinking it—that swirled Instagram look comes from restraint, not effort. Drop the peanut butter in clumps rather than spreading it all the way, then make just a few passes with your knife to create those organic lines. If you swirl too much, the peanut butter breaks apart and distributes unevenly, and you lose those gorgeous streaks of contrast.

Storage and Serving

These brownies actually taste better the next day once the flavors have melded together, so don't feel pressured to eat them fresh from the oven. Store them in an airtight container at room temperature for up to five days, or wrap them individually and freeze for up to a month.

• Warm them in the microwave for 15 seconds before serving if you want that fudgy texture to come alive again.
• A scoop of vanilla ice cream on top turns these from a snack into dessert.
• These also mail well to friends if you wrap them in parchment and then foil—they're sturdy enough to travel.

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Making these brownies has become my way of keeping my sourdough starter alive in a way that actually matters. They remind me that sometimes the best recipes come from solving problems instead of following a plan.

Recipe Questions

→ How does sourdough discard affect the brownies?

Sourdough discard adds moisture and a subtle tang, enriching the brownies' texture and enhancing depth of flavor without overpowering the chocolate.

→ Can I substitute peanut butter in the swirl?

Yes, sunflower seed butter or almond butter can be used as alternatives for a nut-free or different nut flavor variation.

→ What is the best way to achieve a smooth peanut butter swirl?

Drop spoonfuls of peanut butter mixture over the batter and gently swirl with a skewer or knife to create a marbled effect without fully mixing.

→ How should I know when the brownies are done baking?

Check for a slightly set center where a toothpick inserted comes out with moist crumbs; avoid overbaking to maintain fudginess.

→ Can I add toppings or mix-ins?

Adding chocolate chips or chopped nuts before baking provides extra texture and flavor to complement the rich chocolate and peanut butter.