In small bowl, beat powdered sugar and butter with electric mixer on low speed until blended. Stir in 1 tablespoon milk and vanilla. Gradually beat in just enough remaining milk to make frosting smooth and spreadable. Add red food color; beat until well blended and to desired color. Spread frosting on cake, leaving 1/2-inch border around edge of cake. With vegetable peeler, make shavings from white chocolate baking bar. Sprinkle over frosting.