1. Beat butter at medium speed with an electric mixer until fluffy; gradually add sugar, beating well. Add egg yolks, 1 at a time, beating until blended after each addition.
2. Combine flour and baking powder; add to butter mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Stir in vanilla.
3. Beat egg whites at high speed with electric mixer until stiff peaks form; fold one-third of egg whites into batter. Gently fold in remaining beaten egg whites just until blended. Spoon batter into 3 greased and floured 9-inch round cake pans.
4. Bake at 350° for 18 to 20 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans, and cool completely on wire racks.
5. Spread Lemon Filling between layers. Spread Cream Cheese Frosting on top and sides of cake. Sprinkle top and sides with coconut. Garnish, if desired.