Ingredients
1 box Betty Crocker SuperMoist yellow cake mix
Water, vegetable oil and eggs called for on cake mix box
2 containers Betty Crocker® Rich & Creamy vanilla frosting
Betty Crocker red, yellow and blue gel food colors
Tray or cardboard (15x12 inches), covered with wrapping paper and plastic food wrap or foil
12 large marshmallows, cut in half crosswise
Directions
1. Heat oven to 350°F (325°F for dark or nonstick pan). Grease or spray bottom and sides of 13x9-inch pan.
2. Make and bake cake mix as directed on box for 13x9-inch pan. Cool in pan 10 minutes; remove from pan to cooling rack. Cool completely, about 1 hour. Refrigerate or freeze cake about 1 hour or until firm.
3. Meanwhile, in small bowl for each color, tint 1 cup frosting red, 1 cup frosting yellow and 2/3 cup frosting blue with food colors. Leave remaining frosting white.
4. Using serrated knife, cut rounded dome from top of cake to make flat surface; place cake cut side down. Cut cake crosswise into thirds. Cut one of the thirds in half crosswise to make 2 squares. Place cake pieces on tray.
5. To seal in crumbs, frost top and sides of 1 square cake with thin layer of blue frosting and 1 square cake with thin layer of white frosting. Frost 1 rectangular cake with thin layer of yellow frosting and 1 rectangular cake with thin layer of red frosting. Refrigerate or freeze 30 to 60 minutes to set frosting.
6. Add final coat of frosting to each cake. Frost 4 marshmallow halves with blue frosting; place in square design on blue cake. Frost 4 marshmallow halves with white frosting; place in square design on white cake. Frost 8 marshmallow halves with yellow frosting; place on yellow cake. Frost 8 marshmallow halves with red frosting; place on red cake. Store loosely covered.
Here is a video that shows how to do this Cake.
http://www.bettycrocker.com/recipes/building-blocks-cakes/ca36541e-624c-48af-ab78-68ef256e8903?sc=Birthday%20Cake%20Recipes&term=Birthday-Cakes&itemId=aad376c0-d7d5-44a6-b393-8669b73a3c37