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1
Preheat oven to 325F.
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2
Butter an 8-inch square baking pan and line with parchment, allowing a 2-inch overhang.
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3
Butter lining (not overhang).
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4
Make batter: Put 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 flour, baking powder, and salt in a bowl.
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7
Whisk granulated sugar into chocolate mixture.
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8
Add eggs, and whisk until mixture is smooth.
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9
Stir in vanilla.
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10
Add flour mixture; stir until well combined.
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11
Make filling: Stir together butter, confectioners sugar, peanut butter, salt, and vanilla in a bowl until smooth.
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12
Pour one-third of batter into prepared pan; spread evenly with a rubber spatula.
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13
Drop dollops of peanut butter filling (about 1 tablespoon each) on top of batter, spacing about 1 inch apart.
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14
Drizzle remaining batter on top, and gently spread to fill pan.
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15
Drop dollops of remaining peanut butter mixture on top.
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16
Gently swirl peanut butter filling into batter with a butter knife, running the knife lengthwise and crosswise through layers.
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17
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.
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18
Let cool slightly in pan, about 15 minutes.
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19
Lift out; let cool completely on a wire rack before cutting into squares.
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20
Brownies can be stored in an airtight container at room temperature up to 3 days.
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21
Use two large spoons to drop dollops of filling over a base of chocolate batter.
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22
Add more batter on top, then more dollops of filling.
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23
Drag a butter knife back and forth with the tip touching the bottom of the pan.
