This is a classic Christmas white cake in every way, with three golden yellow layers and a fluffy seven-minute frosting. The homemade lemon filling is a quick and easy stovetop stir-together that can also be used to fill tart shells.
Beat butter at medium speed with an electric mixer until fluffy; gradually add sugar, beating well. Add egg yolks, one at a time, beating after each addition.
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 1 tsp. vanilla.
Beat egg whites and salt at high speed with an electric mixer until stiff peaks form. Stir about one-third of egg whites into batter; fold in remaining egg whites. Spoon batter into 3 greased and floured 9-inch round cake pans.
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.
Spread Lemon Filling between layers. Spread Fluffy White Frosting on top and sides of cake. Sprinkle top and sides with coconut; garnish, if desired.
Note: Boxwood is poisonous if ingested. If boxwood sprigs are used as garnish, cut small circle of heavy-duty plastic wrap, and position in center of cake before arranging sprigs and cranberries on top of cake. Remove garnish before serving.