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 think. Fill paper-lined muffin cups two-thirds full. Bake 18 to 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.
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 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.
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.
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 for 1 minute. Continue beating on low over low heat until frosting reaches 160 F, about 8 to 10 minutes.
Pour into a large bowl; add vanilla. Beat on high until stiff peaks form, about 5 minutes. Add butter, 1 tablespoon at a time, beating well after each addition. If mixture begins to look too hot or 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 to 2 minutes or until smooth.
Place frosting in a pastry 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.
