To get a smooth surface, you may need to refrigerate the cake before putting the final white layer on, depending on the temperature of the room you are working in. Use the remaining white frosting to frost the outside of the cake using an offset spatula and cake smoothing comb or bench/dough scraper. On a rotating cake stand, add a cardboard cake circle, add the first layer (red), add a little frosting to make a thin layer between the cake layers, add the 2nd layer (orange), and repeat with yellow, green, and purple cake layers.Add each bowl of frosting into piping bags with desired frosting tips. Add gel food coloring, and mix until the color is even. Divide the buttercream frosting into 3 equal bowls (or however many colors you wish to do).Half will be used for frosting the cake base and half will be further divided into colors for decorating the unicorn mane. Make sure the frosting is completely smooth before mixing gel food coloring in. Gradually add the confectioner's sugar, ½ cup at a time.Add vanilla and milk, and mix until combined.In a stand mixing bowl, add unsalted butter and beat for about 30 seconds.Make sure the cake layers are completely cool before frosting. When the cake layers are done, cool on a wire rack for 5-10 minutes before removing from the pans on a wire rack to finish cooling completely.Bake for 17-20 minutes, rotating halfway during the baking time.(Alternately, you can mix the colors in separate bowls if you don't want to mix them in the cake pans.) Add drops of the gel food coloring to the 5 cake pans and gently stir to combine.Add the butter evenly to each of the 5 baking pans. Add the flour mixture and buttermilk alternately to the butter mixture gradually.Add the egg whites gradually, one at a time beat well after adding each one. Add the granulated white sugar and 1 teaspoon of pure vanilla extract. In a large mixing bowl, beat butter with a handheld electric mixer for about 30 seconds.

In a large mixing bowl, combine the all-purpose flour, baking powder, baking soda, and Kosher salt.Grease the 5 round cake pans and set them aside.I continue washing the spatula in the hot water over and over until I like the results. Then with it clean of frosting, I run it over the frosting layer to give it a smooth finish. Right after I have frosted the entire cake, I take my flat spatula and dip it into a tall cup of hot water. Make sure to go easy on the frosting between layers so you have enough to cover the entire cake. This is just enough for frosting this BIG cake. This makes a very light and creamy frosting.

Take your cake out and frost the entire cake. This will seal your cake crumbs so that they will not stick to your final frosting layer. Place your cake back into the freezer for another hour. I forgot to take a picture of this step, sorry. This layer will have a lot of crumbs in it, but don’t worry, they will not show. Then take your frosting and lightly frost the entire cake. Have you ever had that problem? After you have your cake layers placed together, before you frost the rest of the cake, place your entire cake into the freezer for about an hour or so. I have learned the trick for getting a nice clean frosting layer without all the cake crumbs in it. I baked the cakes in a 350 degree oven for about 12 minutes. Be sure to grease your pans well so that the cakes come out without sticking. I washed the pans and baked the other 3 cakes. I only had three, so I baked 3 first and then when they had cooled, I transferred the cakes to a cookie sheet. When each color is mixed, place batter into 6 separate round 9 inch pans. Continue in this manner until you have added all your cake layers. Place your blue cake layer on top of the frosting. After all the cake layers have cooled, place your bottom layer (purple) onto your cake plate.

Rainbow colors cake