Mix cake batter according to directions on the box.

Put 1 1/2 cups of cake batter into 3 separate bowls and add food coloring to make pink, blue and yellow cake batter.

Pour into greased cake pans and bake at 350 degrees for 25-30 minutes for 4-inch springform pans or 15-20 minutes for 6-inch cake pans. Bake until an inserted toothpick comes out clean.

Beat butter, shortening, and vanilla for two minutes, until creamy.

Add powdered sugar, 1 cup at a time, beating after each cup.

Beat in 1/4 - 1/2 cup of heavy cream until light and fluffy. If necessary, add additional heavy cream, 1 tablespoon at a time until it is the consistency you want.

Once the cakes have cooled completely, spread a little frosting in the center of the cake platter and place one of the cakes on the frosting. Spread about 1/3 cup of buttercream between the layers.

Cover the entire cake with a thin layer of frosting, making a crumb coat which will seal the crumbs in place.

Once the crumb coat frosting is dry to the touch, frost the cake with the buttercream frosting.

Press Cadbury Mini Eggs into the buttercream frosting making a pattern.