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1
Adjust rack to middle position and preheat oven to 350u00b0F. Grease 8- by 8-inch baking pan and line with 8- by 14-inch piece of parchment paper, allowing both ends to overhang the edge of the pan.
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2
In microwave or over pan of barely simmering water, heat chocolate and butter, stirring, until melted and smooth. Whisk in sugar, brown sugar, and salt. Whisk in peanut butter, eggs, yolk, and vanilla until smooth.
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3
Sift flour and cocoa powder into small bowl. Stir into chocolate mixture until completely combined. Pour into prepared pan and smooth the top. Bake until just set and toothpick inserted into center comes out with moist crumbs, 20 to 25 minutes. Transfer to wire rack to cool completely before topping with peanut butter layer.
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4
Meanwhile,
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5
: In standing mixer fitted with paddle attachment (or hand held mixer), beat peanut butter and butter on medium low speed until creamy. Beat in confectioners' sugar, vanilla, and salt until smooth and creamy. Spoon into pan over brownies and smooth the top. Chill while making ganache topping.
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6
: Place chocolate, sugar, and peanut butter in medium bowl. Heat cream in microwave or on stove until just simmering. Whisk into chocolate mixture until smooth. Pour over peanut butter layer (see note). Chill until firm, 2 to 3 hours.
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7
To cut, run sharp knife along inside edge of pan. Grab parchment paper, lift brownies out of pan, and place on work surface. Use a large hot, wet knife to cut into squares.
