For this cake, you'll need to make five cake layers: two white, two red, and one blue. You will also need three batches of icing. I made vanilla buttercream. After cutting some of the layers, you will end up with a four layer cake once you are finished. I used my trusty Funfetti cake mix for a vibrant red and blue color.
Bake the cakes as directed. (Each of these particular cake mixes call for egg whites, so I purchased a carton of liquid egg whites.)
Once they have cooled for an hour, freeze them so they will be easier to cut. I just loosely covered each layer with aluminum foil, then stacked them directly on top of each other. If you freeze them overnight, you would need to cover them layers more tightly.
Level frozen cakes with a serrated knife or cake leveler. Since there will be so many layers to stack, you don't want any domes.
Cut each red and white layer in half so you will have four thin red layers and four thin white layers. If your cakes aren't firm, you might need to use a large spatula or cake lifter so they don't crack in your hands.
Place a thin layer of icing between one of the red and one of the white layers. Place this back in the freezer. You will be cutting the edge off soon and need it to be firm.
Use a bowl, smaller cake pan, or any other circle as a template. Place it in the middle of the blue cake layer. If the blue cake is frozen, it will be very sturdy and easy to cut. You will only need the outer ring for the flag cake, so you can set the small blue circle aside.
Retrieve the red and white layer from the freezer. Use the same round pan and place it in the middle of the red and white layer. Cut around it and remove the inner circle. This time you will not be using the outer ring, so set it aside.
Apply a thin coat of icing along the inside of the blue ring. Place the small red and white circle inside the blue ring.
From this point onward, you simply frost and stack the layers, alternating red and white. I had a leftover white layer that was especially thin that I chose not to use. Plus, the U.S. flag begins and ends with a red stripe, so I liked having a red layer on top and bottom.
Here is the final cake before is it frosted:
Apply a crumb coat of icing and place it in the fridge or freezer to set.
Finish the cake with a final coat of icing. You could decorate the outside with sprinkles, stars, etc. I chose to leave it plain since I wanted the focus of the cake to be the inside. Here is what it looked like once it was sliced open.
The moderate amount of effort required to make this cake is definitely worth the surprise inside!