Preheat oven to 350 degrees and butter and flour a 9-inch square baking pan, knocking out excess flour. In a double boiler or metal bowl set over a saucepan of barely simmering water melt chocolate, stirring, until smooth and remove top of double boiler or bowl from heat. Cool chocolate to room temperature.
In a large bowl with an electric mixer beat butter and sugar until light and fluffy and beat in eggs, 1 at a time, beating well after each addition, and vanilla. Into a small bowl sift together flour, baking powder, and salt and beat into butter mixture just until blended well. Divide batter between 2 bowls. Whisk peanut butter into batter in 1 bowl. Whisk melted chocolate into batter in other bowl and stir in chopped chocolate or chocolate chips.
Spread peanut-butter batter evenly in pan. Drop chocolate batter by large spoonfuls onto peanut-butter batter and spread carefully to form an even layer. Bake brownies in middle of oven 35 to 40 minutes, or until a tester comes out with crumbs adhering to it. Cool brownies completely in pan on a rack before cutting into 16 squares. Brownies keep, layered between sheets of waxed paper in an airtight container at cool room temperature, 5 days.
Recipe courtesy of Gourmet Magazine