½ cup unsalted butter, cut into small pieces, plus more for pan 2 ounces unsweetened chocolate, coarsely chopped 4 ounces semisweet chocolate, coarsely chopped 2/3 cup all-purpose flour ½ teaspoon baking powder ¼ teaspoon coarse salt ¾ cup granulated sugar 3 large eggs 2 teaspoons pure vanilla extract
For the filling:
4 tablespoons unsalted butter, melted ½ cup confectioners sugar ¾ cup smooth peanut butter ¼ teaspoon coarse salt ½ teaspoon pure vanilla extract
1. Preheat oven to 325˚F. Butter an 8-inch square baking pan and line with parchment, allowing a 2-inch overhang. Butter lining (not over-hang).
2. Make batter: Put butter and chocolates in a heatproof medium bowl set over a pan of simmering water; stir until melted. Let cool slightly. Whisk together flour, baking powder, and salt in a bowl.
3. Whisk granulated sugar into chocolate mixture. Add eggs, and whisk until mixture is smooth. Stir in vanilla. Add flour mixture; stir until well combined.
4. Make filling: Stir together butter, confectioners sugar, peanut butter, salt, and vanilla in a bowl until smooth.
5. Pour one-third of batter into prepared pan; spread evenly with a rubber spatula. Drop dollops of peanut butter filling (about 1 tbsp each) on top of batter, spacing about 1 inch apart. Drizzle remaining batter on top, and gently spread to fill pan. Drop dollops of remaining peanut butter mixture on top. Gently swirl peanut butter filling into batter with a butter knife, running the knife lengthwise and crosswise through the layers.
6. Bake until a cake tester inserted into brownies comes out with a few crumbs, but not wet, about 45 minutes. Let cool slightly in pan, about 15 minutes. Lift out; let cool completely on a wire rack before cutting into squares. Brownies can be stored in an airtight container at room temperature up to 3 days.