Cream butter; gradually add sugar, beating well. Add egg yolks, one at a time, beating well after each addition.
Combine flour, baking powder, and salt; add to creamed mixture alternately with milk, beginning and ending with flour mixture. Stir in vanilla.
Beat egg whites (at room temperature) until stiff peaks form; fold into batter.
Spoon batter into a greased and floured 13 1/2- x 9 1/2-inch book-shaped cake pan. Bake at 350° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool cake in pan 10 minutes; remove from pan, and let cool completely.
Spread frosting on top and sides of cake, reserving 1/2 cup frosting. Tint reserved frosting bright blue. Spoon tinted frosting into a pastry bag. Pipe a Star of David and desired inscription on top of cake.