Wow! Red, white and blue cake just in time for the Fourth! What could say America better than this? Lots of prep involved but well worth it for the effect it has!
Combine cake mixes, egg whites, oil, milk, and vanilla extract in a large mixing bowl. Using an electric mixer on low speed, beat for 30 seconds. Increase mixer speed to medium and beat for 2 more minutes. The batter will be slightly lumpy.
Measure half of batter into a separate bowl and whisk in red food coloring. Divide red batter among 2 of the prepared pans (4 greased 8-inch round cake pans lined with parchment paper); smooth tops with spatula. Transfer half of white batter into the third prepared cake pan; smooth top. Whisk blue food coloring into remaining batter; transfer to remaining prepared pan, and smooth top.
Bake cakes in a preheated 350º oven until a toothpick inserted into centers comes out clean, 30 to 35 minutes. Set pans on racks and cool completely.
Run a knife around edge of each cooled cake. Invert cakes, remove pans, and peel off parchment paper. Trim tops of cakes flat and even. Cut red and white cakes in half horizontally to make 4 thin red-cake layers and 2 thin white-cake layers.
Using stencil as a guide, cut out 4-inch rounds from centers of blue cake, 1 thin white layer, and 1 thin red layer. Set aside an 8-inch white ring, 4-inch blue round, 8-inch red ring, and 1 thin red layer for another use. Or crumble them into separate bowls and use to decorate outside of finished cake, if desired.
On a cake plate, layer 1 thin red layer, 1 thin white layer, 1 thin red layer, and thick blue ring. Frost top of each layer before adding next layer. Carefully frost inside of blue cake ring. Gently fit thin 4-inch white round into blue ring, and frost top. Set thin 4-inch red round layer on top of white one; gently press flush with top of blue ring. Frost top and sides of cake generously with remaining frosting.
If desired, sprinkle stripes or star patterns on top of cake using reserved cake crumbs. Chill cake 1 hour for easiest slicing.
You may skip the white cake layer and just do all red and blue. The white frosting acts as the white stripes.
Make sure cake is cooled COMPLETELY before serving or it will crumble when sliced.