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1
Preheat the oven to 325F.
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2
Line a buttered 8-inch-square baking pan with foil or parchment paper, allowing 2 inches to hang over the sides.
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3
Butter the lining (excluding the overhang); set the pan aside.
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4
Make the batter: Put the butter and chocolates in a heatproof medium bowl set over a pan of simmering water; stir until melted.
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5
Let cool slightly.
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6
Whisk together the flour, baking powder, and salt in a separate bowl, and set aside.
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7
Whisk the granulated sugar into the chocolate mixture.
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8
Add the eggs, and whisk until the mixture is smooth.
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9
Stir in the vanilla.
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10
Add the flour mixture; stir until well incorporated.
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11
Make the filling: Stir together the butter, confectioners sugar, peanut butter, salt, and vanilla in a bowl until smooth.
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12
Pour one-third of the batter into the prepared pan, and spread evenly with a rubber spatula.
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13
Place dollops of filling (about 1 tablespoon each) about 1 inch apart on top of the batter.
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14
Drizzle the remaining batter on top, and gently spread to fill the pan.
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15
Place dollops of the remaining filling on top.
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16
With a butter knife, gently swirl the filling into the batter, running the knife lengthwise and crosswise through the layers.
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17
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.
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18
Let cool slightly in the pan, about 15 minutes.
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19
Lift out the brownies; let cool completely on a wire rack before cutting into squares.
