Peanut Butter Swirl Brownies

Source: Martha Stewart Holiday Cookies

For the batter:
1/2 cup (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, coarsley chopped
2/3 cup all-purpose flour
1/2 teaspoon baking powder
1/4 teaspoon coarse salt
3/4 cup sugar
3 large eggs
2 teaspoons pure vanilla extract

For the filling:
1/4 cup (1/2 stick) unsalted butter, melted
1/2 cup confectioners' sugar
3/4 cup smooth peanut butter
1/4 teaspoon coarse salt
1/2 teaspoon pure vanilla extract

1. Preheat oven to 325º. Line a buttered 8-inch square baking pan with foil or parchment paper, allowing a 2-inch overhang. Butter lining; set pan aside.
2. Make batter: Put butter and chocolates in a heatproof bowl set over a pan of simmering water; stir until melted. Let cool slightly. Set aside.
3. Whisk together flour, baking powder, and salt in a medium bowl, and set aside.
4. Whisk sugar into chocolate mixture. Add eggs, and whisk until mixture is smooth. Stir in vanilla. Add flour mixture; stir until well combined.
5. Make filling: Stir together butter, confectioners' sugar, peanut butter, salt, and vanilla in a medium bowl until smooth.
6. 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 reamining batter on top, and gently spread to fill pan. Drop dollops of remaining peanut butter mixture on top.
7. 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 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.

I talked about this recipe HERE.