Rainbow Birthday Cake with Sprinkle Buttercream

Today’s fun and colorful rainbow cake is perfect for birthday parties! In fact, it’s the cake my youngest son requested last week for his third birthday, and my boys absolutely loved getting to help bake and assemble all the colorful cake layers. With a demure coat of vanilla buttercream flecked with sprinkles, it’s so fun to cut the first slice and reveal the vivid rainbow hiding inside.

So let’s talk about the fact that this rainbow cake has 6 layers, but a rainbow actually has a spectrum of 7 colors: red, orange, yellow, green, blue, indigo and violet. The main reason I just made 6 layers, though, is because I only have 6 cake pans. That said, you can certainly bake the batter in batches if you don’t have enough pans. So if you want to squeeze in that indigo layer, then go for it!

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Ingredients

Cake

• Cake Flour. With a lower percentage of protein, cake flour makes a softer, more tender cake crumb than all-purpose flour, and gives the cake strength and structure.
• Granulated Sugar. Adds sweetness and moisture.
• Baking Powder. Leavens the cake.
• Coarse Kosher Salt. Balances the sweetness.
• Eggs + Egg White. Adds structure to the cake batter.
• Buttermilk. Gives the cake moisture and acidity, for a more tender cake crumb.
• Unsalted Butter + Vegetable Oil. A combination of melted butter and oil makes cakes that are really moist, with a nice buttery flavor.
• Vanilla Extract. Flavor
• Gel Food Coloring: red, orange, yellow, green, blue, (indigo) and violet/purple

Today’s photographed rainbow cake is a 6-inch cake, that I scaled down from my favorite vanilla cake recipe. To make an 8-inch cake, use the full recipe found here.

Buttercream

• Unsalted Butter. Butter gives buttercream it’s rich, creamy buttery flavor. For a whiter buttercream, substitute half of the butter with vegetable shortening.
• Powdered Sugar. Sweetens and thickens the buttercream.
• Meringue Powder. A little known secret, meringue powder adds stability and improves the texture of buttercream.
• Vanilla Extract. Flavor.
• Coarse Kosher Salt. Balances the sweetness.
• Sprinkles. After crumb coating the cake, I added rainbow nonpareils to the final coat of buttercream, so that it was flecked all over with color.

Instructions

Cake

• Preheat the oven to 350F and spray the bottoms of six 6-inch cake pans with non-stick spray.
• In a large bowl, sift together the flour, sugar, baking powder and salt. Whisk to combine well.
• In another large bowl, whisk together the eggs, egg whites, buttermilk, melted butter, oil and vanilla extract until smooth.
• Add the dry ingredients to the wet, and whisk until mostly combined and smooth, about 10-15 seconds. Don’t over-mix, since you’ll be mixing the batter again when you add the food coloring.
• Divide the batter evenly between 6 small bowls. Tint each bowl of batter separately with red, orange, yellow, green, blue and purple gel food coloring. Use 1-2 drops for more vivid colors, or use a toothpick to add just a tiny bit of color for pastel shades. Stir the coloring into the batter until combined.
• Scrape the 6 colors of batter into the prepared pans. Bake on the center oven rack for about 13-15 minutes, until the centers spring back when lightly touched. Since there’s a small amount of batter in each pan, the layers will be fairly thin.
• Cool the cakes in the pans before assembling and frosting.

TIP: If you don’t have 6 cake pans, you can bake the batter in batches. Let the extra cake batter rest at room temperature, until you can wash the pans after baking the first batch.

Buttercream

• In the bowl of your stand mixer fitted with the whisk attachment, beat the butter for 1 minute until smooth.
• With the mixer running on low, add the powdered sugar by spoonfuls, the meringue powder and the salt, mixing until thick and combined.
• Add the vanilla and the milk; increase speed to medium and whip for 4-5 minutes, scraping the bowl down several times, until light and fluffy.

Assembly

• Remove the cooled cakes from the pans.
• Set the purple cake on a cake board. Frost with a thin layer of buttercream. Repeat stacking and filling with the remaining colors, ending with red on top.
• Frost the cake all over with a thin crumb coat of buttercream, and chill for 30 minutes.
• Fold the sprinkles into the remaining buttercream, and frost the cake all over with a final coat of buttercream.
• Fit a piping bag with tip 6B, and fill with the leftover buttercream. Pipe the buttercream in decorative peaks on top of the cake.

Frequently Asked Questions

What if I don’t have 6 cake pans?

Since I do so much baking, I have six 6-inch cake pans and six 8-inch cake pans. But I realize a lot of home bakers don’t have so many spare pans. If you only have 2 or 3 pans, you can bake the cake batter in batches. While baking the first batch, let the bowls of colored batter rest at room temperature. When you have a clean pan available, gently scrape the batter into the, trying not to mix it again or disturb it too much, and bake the remaining cake layers. Since baking powder is double acting, you should still get a rise from your cake even though some of the batter wasn’t baked immediately.

How do I make this into a larger 8-inch cake?

For my 6-inch cake, I scaled the recipe down to 2/3 the size of a standard cake. To bake this in six 8-inch cake pans, use my vanilla cake recipe found here.

Can I use all-purpose flour instead of cake flour?

Cake flour contains a lower percentage of protein than other flours, which is why it makes such light, fluffy cakes. You can use all-purpose flour instead.

What can I substitute for the buttermilk?

You can use either whole buttermilk or low fat buttermilk. To make your own buttermilk, add 2 tablespoons white distilled vinegar to whole milk, and warm briefly in the microwave to curdle the milk.

What kind of sprinkles did you use in the buttercream?

I used rainbow nonpareils in the buttercream.

What’s the best food coloring to use?

I recommend using gel food coloring, rather than liquid, for the most concentrated, vivid colors.

What piping tip did you use for the buttercream on top of the cake?

Tip 6B.

Is this a high altitude cake recipe?

Yes, I test all the cakes on my site at Denver’s altitude of 5,280 feet.

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