Instructions

• Preheat oven to 350°F. Line a 9x5-inch loaf pan with parchment paper, leaving an overhang on two opposite long sides to lift the brownies out later. Do not grease the pan.
• If your 1/4 cup (2 oz) unsalted butter is not yet melted, melt it and let it cool until it is warm but not hot.
• In a medium bowl whisk together 1/2 cup (100g) granulated sugar, the cooled melted 1/4 cup (2 oz) unsalted butter, 1 large egg, and 1/2 teaspoon vanilla extract until smooth and combined.
• Add 3 tablespoons (15g) unsweetened cocoa powder (sift if lumpy), 1/4 cup (30g) all-purpose flour, 1/8 teaspoon baking powder, and 1/8 teaspoon salt to the bowl. Stir gently until just combined and no large streaks of dry ingredients remain.
• Transfer the batter to the prepared loaf pan and spread it evenly with a spatula or knife so it mostly covers the bottom of the pan.
• In a small bowl whisk together 1 tablespoon (.5 oz) unsalted butter (melted), 2 tablespoons (15g) powdered sugar, 3 tablespoons smooth peanut butter, 1/8 teaspoon vanilla extract, and a pinch of salt until smooth and combined.
• Drop the peanut butter mixture by spoonfuls (about 6–8 dollops) evenly over the brownie batter.
• With a small knife, swirl the peanut butter into the batter: draw three parallel lengthwise lines down the long side of the pan through the dollops, then drag the knife in an “S” shape across those lines to create a marbled swirl. Work gently—do not fully blend.
• Bake in the preheated 350°F oven for 22 to 25 minutes, until a toothpick inserted into a non-peanut-butter section near the center comes out with just a few moist crumbs.
• Remove the pan from the oven and set it on a cooling rack. Cool in the pan for at least 15 minutes.
• Use the parchment overhang to lift the brownies from the pan, slice, and serve.