Preheat oven to 350°.
Combine flour, cocoa, baking powder, and salt in a bowl; set aside.
Combine sour cream, butter, and vanilla in a medium bowl. Add sugar; beat with a mixer at medium speed until combined. Add egg whites and egg; beat well. Stir in flour mixture. Pour batter into a lightly greased 9-inch square baking pan. Bake at 350° for 20 to 25 minutes or until a wooden pick inserted in center comes out almost clean.
Spread caramel dip over brownie. Top evenly with marshmallows and nuts. Pour warm Double Chocolate Frosting evenly over marshmallows. Cool in pan on a wire rack.