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1
Preheat oven to 350 degrees and butter and flour a 9-inch square baking pan, knocking out excess flour.
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2
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.
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3
Cool chocolate to room temperature.
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4
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.
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5
Into a small bowl sift together flour, baking powder, and salt and beat into butter mixture just until blended well.
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6
Divide batter between 2 bowls.
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7
Whisk peanut butter into batter in 1 bowl.
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8
Whisk melted chocolate into batter in other bowl and stir in chopped chocolate or chocolate chips.
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9
Spread peanut-butter batter evenly in pan.
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10
Drop chocolate batter by large spoonfuls onto peanut-butter batter and spread carefully to form an even layer.
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11
Bake brownies in middle of oven 35 to 40 minutes, or until a tester comes out with crumbs adhering to it.
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12
Cool brownies completely in pan on a rack before cutting into 16 squares.
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13
Brownies keep, layered between sheets of waxed paper in an airtight container at cool room temperature, 5 days.
