Peanut Butter Swirl Brownies

Martha Stewart Living

Makes 9 large or 16 small squares

    BATTER:                                                        
    1 stick unsalted butter cut to pieces +M4 pan                  
    2 ounces unsweetened chocolate bar coarse chop                 
    4 ounces semisweet chocolate bar coarse chop                   
    2/3 cup flour                                                  
    1/2 teaspoon baking powder                                     
    1/4 teaspoon salt                                              
    3/4 cup sugar                                                  
    3 eggs                                                         
    2 teaspoons vanilla extract       
                             
    FILLING:                                                       
    4 tablespoons unsalted butter melted                           
    1/2 cup confectioners' sugar                                   
    3/4 cup creamy peanut butter                                   
    1/4 teaspoon salt                                              
    1/2 teaspoon vanilla extract                                   

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

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 separate bowl, and set aside.

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

FILLING:
Stir together butter, confectioners' sugar, peanut butter, salt, and vanilla in a bowl until smooth.

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

With a butter knife, gently swirl filling into batter, running 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 brownies; let cool completely on a wire rack before cutting into squares.