Peanut Butter Brownies

Preparation

1. Step 1

   Place an a rack in middle of oven and preheat to 325°. Lightly coat an 8x8" metal baking pan with nonstick vegetable oil spray, then line with 2 strips of parchment paper, running strips perpendicular to one another and leaving a generous overhang on all sides. Whisk ¾ cup plus 1 Tbsp. (102 g) all-purpose flour, ⅓ cup (35 g) Dutch-process cocoa powder, and 1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt in a small bowl to combine.

   Step 2

   Combine 10 Tbsp. (1¼ sticks) unsalted butter, cut into ½" pieces, and ⅔ cup semisweet chocolate chips in a large heatproof bowl set over a large saucepan of simmering water. (bottom of bowl should not be touching water) and heat, stirring occasionally, until melted and smooth, 6–8 minutes. Turn off heat but keep bowl on top of pan to keep mixture warm. (Alternatively, combine butter and chocolate in a large microwave-safe bowl and microwave on high in 30-second bursts, stirring between each, until melted and smooth, 1–2 minutes total.)

   Step 3

   Beat 3 large eggs, room temperature, 1 cup (200 g) granulated sugar, ¼ cup (50 g; packed) light brown sugar, and 1 Tbsp. vanilla extract in the bowl of a stand mixer fitted with a paddle attachment on medium-high speed until lightened in color, thick, and batter creates slowly dissolving ribbons when folded back onto itself, about 3 minutes. Reduce speed to low, add melted chocolate mixture, and beat just until combined, about 30 seconds. (Chocolate mixture needs to be warm when added, reheat 15–30 seconds in microwave if needed.) Turn off mixer, scrape down sides of bowl, and stir in any chocolate at the bottom of bowl. Add dry ingredients and mix on low speed until almost completely incorporated and a smooth batter forms, about 30 seconds. Using a rubber spatula, stir batter to incorporate any remaining dry ingredients, then scrape batter into prepared pan. Smooth into an even layer with an offset spatula or a spoon. Chill in freezer 10 minutes.

   Step 4

   Dollop ⅓ cup plus 2 Tbsp. creamy peanut butter over batter in about 1 Tbsp. amounts, then swirl into top of batter with a toothpick or paring knife to create a marbled effect. (For a cleaner look, transfer peanut butter to a resealable plastic bag. Close and snip the end off a corner. Pipe over batter, then swirl with a toothpick.)

   Step 5

   Bake brownie until surface is set and edges are firm and starting to pull away from pan, 34–42 minutes. (A tester inserted into the center will have several fudgy crumbs attached but they should not look raw.) Let brownie cool.

   Step 6

   When ready to serve, cut brownie into a 4x4 grid with a serrated knife, wiping the blade clean with a paper towel between slices, to make 16 squares.

   Do ahead: Brownies can be baked 5 days ahead. Store airtight at room temperature.