Preheat oven to 325°F. Prepare pans with vegetable pan spray.
In medium bowl, sift together flour, baking powder and salt; set aside. In large bowl, beat butter and sugar with electric mixer until light and fluffy. Add egg whites one at a time, beating well after each addition. Add vanilla and beat well. Add flour mixture to butter mixture alternately with milk; beat until just combined. If desired, tint portions of cake batter with icing color. Divide evenly into prepared pans.
Bake 17-20 minutes or until toothpick inserted in center of cake comes out clean. Cool 5 minutes in pan on cooling grid; remove and cool completely before icing.
For icing, beat shortening and butter in large bowl with electric mixer until light and fluffy. Beat in vanilla. Gradually add sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been mixed in, icing will appear dry. Gradually add milk; beat at medium speed until light and fluffy.
To assemble, cut the crowns off of each cake layer. Place first layer on cake board or serving plate. Spread a layer of icing to edges of cake. Top with second cake. Repeat with remaining icing and cake. Ice top and sides of cake.