Ingredients:
Batter:8 tablespoons (1 stick) unsalted butter, cut into small pieces, plus more for pan
6 ounces good-quality semisweet chocolate, coarsely chopped
2/3 cup all-purpose flour 1/2 teaspoon baking powder
1/4 teaspoon salt
3/4 cup granulated sugar
3 large eggs
2 teaspoons pure vanilla extract
Filling:
4 tablespoons unsalted butter, melted
1/2 cup confectioners’ sugar
3/4 cup smooth peanut butter
1/4 teaspoon salt
1/2 teaspoon pure vanilla extract
Directions:
Preheat oven to 325 degrees. Butter an 8-inch square baking pan and line with parchment, allowing a 2-inch overhang. Butter lining (not overhang).
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.
Whisk granulated sugar into chocolate mixture. Add eggs, and whisk until mixture is smooth. Stir in vanilla. Add flour mixture; stir until well incorporated.
Make filling: Stir together butter, confectioners’ sugar, peanut butter, salt, and vanilla in a bowl until smooth.
Pour one-third of batter into prepared pan; spread evenly with a rubber spatula. Drop dollops of peanut butter filling (about 1 tablespoon 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 filling on top. Gently swirl peanut butter filling into batter with a butter knife, running the knife lengthwise and crosswise through layers.
Bake until a toothpick inserted into brownies comes out with a few crumbs but is 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
