1 cup butter, cubed
5 ounces bittersweet chocolate, chopped
2 cups sugar
1 1/3 cups all-purpose flour
1/2 cup cocoa powder
1/2 teaspoon salt
4 pieces eggs
2 teaspoons vanilla
PEANUT BUTTER FILLING
1/4 cup butter
1/2 cup peanut butter
1/2 cup confectioner's sugar
1 teaspoon vanilla
1. Preheat oven to 325 degrees. Butter an 9-inch square baking pan and line with parchment, allowing a 2-inch overhang.
2. Make brownie batter: Put butter and chocolate in a heatproof medium bowl set over a pan of simmering water; stir until melted. Let cool slightly.
3. Whisk together flour, cocoa powder and salt in a bowl.
4. Whisk granulated sugar into chocolate mixture. Add eggs, and whisk until mixture is smooth. Stir in vanilla.
5. Add flour mixture; stir until well incorporated.
6. Make filling: Stir together butter, peanut butter, confectioners' sugar, and vanilla in a bowl until smooth.
7. Pour half 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.
8. 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 knife or spatula, running the knife lengthwise and crosswise through layers.
9. Bake until a cake tester inserted into brownies comes out with a few crumbs but is not wet, about 45 minutes. Allow to cool completely on a wire rack before cutting into squares.