Mix together your wet ingredients if you’re baking from scratch or using a box cake mix. This will ensure your eggs are properly beaten and incorporated throughout your batter evenly.

Mix your dry ingredients together if you’re cooking from scratch. Or add your dry cake mix to your bowl of wet ingredients and mix it together.

Separate your cake batter into 5 smaller bowls, and use food coloring to color them red, orange, yellow, green and blue. Add bright, high quality food coloring like Wilton or Americolor brand products that will withstand the heat of baking and remain colorful when they come out of the oven.

For 6-inch round baking pans, I used about one cup of cake batter. You can use a food scale and weigh your cake batter to get more precise measurements for perfect layers if you’d like. Take the weight of your batter and divide it by 5, then weigh out each portion into separate bowls before adding food coloring.

Put a circle of parchment paper at the bottom of each round cake pan to ensure it doesn’t stick when you cool the cake and try to remove it. Add one color of cake batter to each pan then bake it at 350 degrees Fahrenheit for 15-17 minutes. It won’t take long to cook since your layers are thin.

While your cake is cooking, make your buttercream frosting by beating together butter, powdered sugar, vanilla and heavy cream (or milk). For one batch of frosting use one cup of room temperature unsalted butter, four cups of powdered sugar, two teaspoons of vanilla and 2-4 tablespoons of cream. Use a stand mixer or hand mixer, you won't be able to do it with muscle and a whisk.

If you want thin layers of frosting between each cake, you could get away with one batch of frosting. For thicker layers like you see in my photos, make two batches of frosting so you don’t run out.

The majority of your frosting will be white for between each layer of cake and on top. You’ll only need about ¼ cup of frosting for each color that’s used to make the rainbow frosting on top. ¼ cup in one bowl for red, another ¼ cup in a separate bowl for orange, and so forth for yellow, green and blue.

Mix your colored frosting well, then put each individual color in a separate piping bag. Lay down plastic wrap on your counter and pipe a 1 inch stripe of each color side by side on the plastic. Roll the plastic up so the colors create a tube, then cut the top of the plastic wrap tube off so the icing can be pressed out of it later. Put this tube of rainbow-colored frosting in a piping bag with a large open star tip and set it aside.

Once your cakes are baked, cool them completely to room temperature. Remove them from their pans and trim the outer edges off to expose the brightly colored cake. Putting your cakes in the fridge or freezer for a few minutes will make them easier to decorate with the frosting.

Put your blue cake on a cake board or plate, then spread an even layer of white buttercream frosting on top. Add the green cake and then repeat for the rest of your rainbow layers with yellow, orange and red.

For the top layer of your rainbow first birthday cake, add a final smooth layer of frosting and add some rainbow sprinkles on top if desired. Then decorate the top edge of the cake with swirls of rainbow frosting. Add a border to the bottom of your cake next to the cake board if desired.

I chose not to add frosting to the outside of this cake for two reasons, to cut down a bit on the amount of sugar/frosting used for this baby smash cake. And also because the colorful edge of this cake is just too pretty to hide.