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1
Preheat the oven to 350 degrees F. Adjust a rack to the center of the oven.
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2
Line an 8-by-8-inch baking pan with parchment or foil, leaving a 2-inch overhang over the edges.
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3
Cover the pan with some nonstick cooking spray.
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4
Melt the butter and 1 cup chocolate chips together in a heavy-bottomed saucepan set over medium heat, stirring constantly.
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5
Remove from heat and cool slightly.
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6
Whisk together the flour, baking powder, and salt in a medium bowl.
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7
Set aside.
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8
Once the chocolate is slightly cool, stir the brown sugar, granulated sugar, eggs, egg yolks, and vanilla into the saucepan until smooth.
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9
Stir the flour mixture into the saucepan until thoroughly combined.
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10
Pour in the remaining cup of chocolate chips.
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11
To make the peanut butter filling: Beat together the butter, confectioners sugar, peanut butter, vanilla, and salt in a medium bowl until smooth.
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12
Pour the chocolate batter into the prepared pan.
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13
Spoon large dollops of the peanut butter mixture evenly across the top of the batter.
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14
Use the tip of a knife or a toothpick to make pretty swirls with the peanut butter throughout the chocolate.
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15
Bake for 40 minutes, or until a toothpick inserted into the center comes out clean.
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16
Remove from the oven, and let cool completely on a wire rack before cutting into squares.
