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1
Make brownies: Preheat oven to 350 degrees F with rack in middle.
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2
Butter an 8- by 8- by 2-inch baking pan, then line bottom of pan with parchment paper and butter parchment.
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3
Beat together butter and sugar with an electric mixer at medium-high speed until mixture is light and fluffy, then add peanut butter and beat until incorporated.
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4
Beat in egg and vanilla.
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5
Reduce mixer sped to low, then mix in flour and salt until just combined.
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6
Mix in chocolate chips (3/4 cups) then spread batter in baking pan, smoothing top.
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7
Bake until brownies are deep golden, puffed on top and a wooden pick inserted in center come out with some crumbs adhering, 30-40 minutes.
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8
Cool completely in pan on a rack, about 1 1/2 hours.
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9
Make ganache: Put chocolate chips (3/4 cups) in a heatproof bowl.
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10
Bring cream to a boil in a small saucepan, then pour over chocolate chips and let mixture stand for one minute.
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11
Gently whisk in butter until it is incorporated, chocolate is melted, and a smooth mixture forms.
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12
Spread ganache on cooled brownies and let stand until set, about 15 minutes.
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13
Brownies keep in one layer in an airtight container three days.
