Store brownies in an airtight container at room temperature for up to 3 days.

　　

Ingredients

Makes 9 large or 16 small squares

　　

For the Batter

8 tablespoons (1 stick) unsalted butter, cut into small pieces, plus more for the 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

　　

Step 1

Preheat the oven to 325°F. Line a buttered 8-inch-square baking pan with foil or parchment paper, allowing 2 inches to hang over the sides. Butter the lining (excluding the overhang); set the pan aside.

　　

Step 2

Make the batter: Put the butter and chocolates in a heatproof medium bowl set over a pan of simmering water; stir until melted. Let cool slightly.

　　

Step 3

Whisk together the flour, baking powder, and salt in a separate bowl, and set aside.

　　

Step 4

Whisk the granulated sugar into the chocolate mixture. Add the eggs, and whisk until the mixture is smooth. Stir in the vanilla. Add the flour mixture; stir until well incorporated.

　　

Step 5

Make the filling: Stir together the butter, confectioners’ sugar, peanut butter, salt, and vanilla in a bowl until smooth.

　　

Step 6

Pour one-third of the batter into the prepared pan, and spread evenly with a rubber spatula. Place dollops of filling (about 1 tablespoon each) about 1 inch apart on top of the batter. Drizzle the remaining batter on top, and gently spread to fill the pan. Place dollops of the remaining filling on top.

　　

Step 7

With a butter knife, gently swirl the filling into the batter, running the knife lengthwise and crosswise through the layers. Bake until a cake tester inserted into the brownies (avoid center and edges) comes out with a few crumbs but is not wet, about 45 minutes. Let cool slightly in the pan, about 15 minutes. Lift out the brownies; let cool completely on a wire rack before cutting into squares.

　　Reprinted with permission from The Martha Stewart Living Cookbook: The New Classics by Martha Stewart Living Magazine, copyright © 2007. Published by Clarkson Potter, a division of The Crown Publishing Group.Buy the full book from Amazon.