Name: Rainbow Smash Cake
Prep time: 60 min
Cook time: 17 min
Total time: 77 min
Ingredients: 1-2 Boxes of white cake mix (depending on pan size, see article for details); 3 Eggs; 1 cup Milk; 1/2 cup Oil (or melted butter); 4 cups Powdered sugar; 2 tsp Vanilla Extract; 1 cup Unsalted Butter; 2-4 Tbsp Heavy Cream (or milk (for thinning frosting)); Rainbow Sprinkles ((optional)); Food Coloring

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If you’re thinking about making a rainbow smash cake for your baby’s first birthday, this is the perfect cake recipe! When my babies were turning one, I wanted them to have cute mini cakes to smash but I didn’t want to pay a fortune for a pro-baker to make them. If you’re in the same boat, why not make one yourself?

Aside from being beginner-friendly, the other great thing about this rainbow smash cake, is it uses a little less frosting than other birthday cakes because it has a ‘live edge’ otherwise known as a ‘naked cake’ because there is no coating on the outer layers. This cuts down a bit on the amount of frosting your baby digs into and it also makes it an easier cake to make for a true beginner.

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Check out this article next for my free 1st Birthday Party Checklist for more tips on hosting your baby’s party! Learn all about how to make a First Birthday Time Capsule in this article next!

Ingredients Needed

Box cake mix: You can use 1-2 boxes of cake mix for this recipe, depending on how big your round cake pans are. For small pans 6 inches or less in diameter, one box should be fine. For larger pans, use two boxes of cake mix. You can alternatively make a recipe for white cake from scratch.

Wet ingredients: Most box cake mixes call for oil or butter. Using butter will give your cake a more rich flavor. You will also need eggs and either water or milk. I suggest using milk instead of water if you can.

Unsalted butter: You’ll need unsalted butter to make the buttercream icing from scratch for this recipe.

Powdered sugar: Powdered sugar is used for the buttercream frosting.

Vanilla extract: Using pure vanilla extract will give your buttercream frosting a better flavor.

Cream: You can use milk or cream for your buttercream frosting to thin it to the desired consistency. I prefer using heavy cream.

Rainbow sprinkles: They are optional to give the top of your rainbow smash cake a more decorative look. Ensure your sprinkles are the tiny kind – no choking hazards here. I usually suggest these rainbow sprinkles.

Food coloring: Use a good quality brand like Wilton or Americolor for vibrant colored cake layers. Unfortunately, some cheaper food coloring brands are all right for coloring frosting but when you bake them, the heat destroys the color and your layers will turn out more brown than bright. I personally use Wilton Color Right drops.

How To Make A Rainbow Smash Cake

Step 1: Mix Wet Ingredients

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.

Step 2: Mix Dry Ingredients

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.

Step 3: Add Food Coloring to Batter

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.

Step 4: Bake Rainbow Cake Layers

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.

Step 5: Mix Frosting

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.

Step 6: Color Frosting

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.

Step 7: Cool & Trim Cake

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.

Step 8: Assemble and Decorate Cake

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.

Rainbow Smash Cake Tips

Batch cook if you don’t have enough pans.
- If you don’t have a set of layer cake pans, just bake your cakes in batches. The thin layers cook quickly and it won’t take too long for them to cool either.
Trim cake layers to make them flat
- If your cake isn’t flat, trim it once it’s cooled so your layers lie smooth. If one spot of your cake is higher than the other, your cake will lean to one side.
Refrigerate or freeze your cake before frosting.
- A chilled cake is easier to put frosting on. It hardens the cake layers enough to make them more resistant to crumbling when you spread icing around.
- Never frost a warm cake that hasn’t cooled completely yet. The icing will melt and turn into a mess.
No choking hazards, gumballs, candles or sparklers near baby.
- This is a smash cake for a baby, so it’s important not to put choking hazards on it like gumballs or candies.
- If you’re putting a candle or sparkler on the cake, do so when you’re holding the cake further away from your child and extinguish it before your child touches the cake for safety purposes.
Smash the cake at the end of the party.
- A smash cake is bound to get messy. Plan on using it as the grand finale to your party and bring a change of clothes. If you’re at home, your baby might need a quick bath, and if you’re not at home bring a lot of wipes, haha.

Rainbow Smash Cake FAQ’s

Can I make a gluten free rainbow smash cake?

Yes, you can use your favorite gluten free cake mix for a rainbow smash cake if needed.

Can you store this rainbow smash cake in the fridge?

Yes, you can store this finished cake on your counter for a day or two, or even longer in your fridge. Just let the cake warm to room temperature before you enjoy eating it. Store it in an airtight container so it doesn’t dry out.

What kind of food coloring gives vibrant colors?

If you bake your cake layers and find that they have turned brown instead of staying colorful, the problem is likely the brand of food coloring you’re using. Use Amerigel or Wilton brand food coloring for vibrant cake layers that won’t be destroyed by oven heat.

Final Thoughts on Rainbow Smash Cakes

Your one year old is going to love this delicious cake, and it’s going to look beautiful for your rainbow baby first birthday dessert table. If you don’t mind a little mess, a smash cake is such a memorable way to help celebrate your baby’s first journey around the sun. This cake is also a great option for a rainbow baby shower cake and you can even use it for St. Patrick’s Day celebrations for a rainbow-themed party.

Your family might also love this raspberry muffin recipe! If you’re still looking for different smash cake ideas, check out this cookie monster smash cake too. Save this recipe for Pinterest for later and share it with someone else who might love this cake.