Prepare Cake: Preheat oven to 350°. Beat butter at medium speed with an electric mixer until creamy; gradually add sugar, beating until light and fluffy.Add egg yolks, 1 at a time, beating until blended after each addition. Stir together flour and next 2 ingredients; add to butter mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed just until blended. Stir in zest and juice. Beat egg whites in a large bowl at high speed until stiff peaks form. Gently stir one-third of egg whites into batter; fold in remaining egg whites. Spoon batter into 4 greased (with shortening) and floured 9-inch round cake pans.Bake at 350° for 16 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 to wire racks, and cool completely.