Dollops of peanut butter filling are spooned onto brownie batter; pulling a knife back and forth through both results in a marbleized look.
FOR THE BATTER
8 tablespoons (1 stick) unsalted butter, cut into small pieces, plus more for pan
2 ounces good-quality unsweetened chocolate, coarsely chopped
4 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
FOR THE 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
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 cake tester inserted into brownies (avoid center and edges) 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. Brownies can be stored in an airtight container at room temperature up to 3 days.
To swirl brownies, work quickly so that batter and filling don’t set. Use two large spoons to drop dollops of filling on the first layer of batter about one inch apart. Repeat with next layer of batter and remaining filling. Drag a butter knife back and forth through the layers with the tip touching the bottom of the pan.
Martha Stewart Living Special Issues, 2005