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1
Preheat oven to 350u00b0. In a large bowl, combine first six ingredients. Whisk buttermilk, coffee, oil, eggs and vanilla until blended; add to the dry ingredients until combined. (Batter will be very thin.) Fill paper-lined muffin cups two-thirds full.
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2
Bake 18-20 minutes or until a toothpick inserted in the center comes out clean. Cool 10 minutes before removing from pans to wire racks to cool completely.
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3
In a small bowl, cream peanut butter, butter, confectioners' sugar and enough milk to achieve piping consistency. Cut a small hole in the corner of a pastry or plastic bag; insert a small round tip. Fill with peanut butter filling. Insert tip into the top center of each cupcake; pipe about 1 tablespoon filling into each.
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4
Place chocolate chips in a small bowl. In a small saucepan, bring cream just to a boil. Pour over chocolate; whisk until smooth. Dip the top of each cupcake into ganache; place on wire racks to set.
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5
In a large heavy saucepan, combine brown sugar, egg whites, salt and cream of tartar over low heat. With a hand mixer, beat on low speed 1 minute. Continue beating on low over low heat until frosting reaches 160u00b0, about 8-10 minutes. Pour into a large bowl; add vanilla. Beat on high until stiff peaks form, about 5 minutes.
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6
Add butter, 1 tablespoon at a time, beating well after each addition. If mixture begins to look curdled, place frosting bowl in another bowl filled with hot water for a few seconds. Continue adding butter and beating until smooth. Beat in peanut butter 1-2 minutes or until smooth.
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7
Place frosting in a pastry or plastic bag with large star tip; pipe onto each cupcake. If desired, top with chocolate curls. Store in an airtight container in the refrigerator. Let stand at room temperature before serving.
