Pour cake batter into 3 greased and floured 8-inch round cakepans.
Bake at 325° for 30 to 35 minutes or until a wooden pick inserted in center of each layer comes out clean. Cool in pans on wire racks 10 minutes; remove from pans, and cool completely on wire racks.
Split layers horizontally. Spread Pina Colada Filling evenly between layers (not on top); cover and chill at least 4 hours.
Spread Rum Buttercream Frosting on top and sides of cake; sprinkle top and sides evenly with flaked coconut, pressing in gently. Garnish, if desired. Store cake in refrigerator. (Cake may be frozen up to 1 month.)