Easy Unicorn Birthday Cake with Rainbow Layers
This unicorn birthday cake has a whole rainbow hiding inside, and it starts with a box of vanilla cake mix so you can skip straight to the fun part. The buttercream is made with real butter, and the decorations are simple items you can find at any grocery store.
A cake that looks this impressive does not need a pastry degree. You mix, color, and stack, then top it with a golden horn and a scatter of sprinkles. It is the kind of project that feels more like an afternoon craft than a baking challenge.
Why This Unicorn Cake Works
The cake layers are soft and moist because I use buttermilk instead of water, and melted butter instead of oil. It tastes like a homemade vanilla cake with a tight, tender crumb.
The buttercream is just sweet enough to balance the cake, and it gets a little tang from a spoonful of sour cream. It spreads smoothly and stays put on the sides, so the rainbow stays hidden until you slice in.
The decorations are unfussy. A sugar cone dipped in gold sprinkles makes the horn, and the ears are two triangles of white chocolate. You can place them however you like and it will look right.
Ingredients
- 1 box (15.25 oz) vanilla cake mix: any standard brand works, avoid the ones with pudding in the mix.
- 1 cup buttermilk: adds tenderness; if you do not have it, stir 1 tbsp lemon juice into 1 cup milk and let it sit 5 minutes.
- ½ cup melted unsalted butter: makes the crumb richer than oil.
- 3 large eggs: bring them to room temperature for a fluffier batter.
- Gel food coloring: red, orange, yellow, green, blue, and purple. Gel gives bright color without thinning the batter.
- 1 ½ cups unsalted butter, softened: for the frosting.
- 4 cups powdered sugar: sift it if it is lumpy.
- ¼ cup sour cream: keeps the frosting from being cloying.
- 2 tsp vanilla extract: use pure extract if you can.
- 1 to 2 tbsp milk: to thin the frosting as needed.
- 1 sugar cone: for the horn.
- Gold sanding sugar or gold sprinkles: to coat the horn.
- White chocolate melting wafers or a bar of white chocolate: for the ears.
- Rainbow sprinkles: a mix of shapes looks best, but any will do.
Variations / Substitutions
- Gluten-free cake mix: swap in a 15.25 oz gluten-free vanilla cake mix and check that your sprinkles and decorations are gluten-free.
- Natural food coloring: powdered fruit and vegetable dyes give pastel layers, though the colors will be softer.
- Different frosting flavor: add the zest of a lemon or a drop of almond extract to the buttercream.
- No sour cream: use an equal amount of full-fat cream cheese that has been beaten smooth first.
- No white chocolate: you can cut ear shapes from a thin sugar cookie or even use large almond slivers.
- Horn alternative: a waffle cone works, just trim the wide end. For a simpler look, you can pipe a tall spiral of frosting instead.
If you want to try the same cake in cupcake form, the vanilla rainbow cupcakes from the same site use the same batter with a shorter bake.
How to Make an Easy Unicorn Birthday Cake
Step 1: Mix the Cake Batter
Heat your oven to 350°F (175°C). Grease two 8-inch round cake pans and line the bottoms with parchment paper circles.
In a large bowl, whisk together 1 box cake mix, 1 cup buttermilk, ½ cup melted butter, and 3 eggs until the batter is smooth and no dry streaks remain. It will look a little thicker than a box mix you make with oil, and that is exactly what you want for bright, even layers.
Step 2: Color the Batter
Divide the batter evenly into 6 small bowls. You can use a cookie scoop or a ladle to keep the amounts even. Each bowl will get about a generous ⅓ cup of batter.
Add a few drops of one gel color to each bowl: red, orange, yellow, green, blue, and purple. Stir each one gently until the color is even and no white streaks show. The batter will look vibrant, and it gets slightly brighter in the oven.
Step 3: Bake the Layers
Spoon the red batter into one prepared pan, spreading it gently with the back of the spoon to cover the bottom. Bake on the middle rack for 12 to 14 minutes, until the top springs back when you press it lightly.
Let the layer cool in the pan for 5 minutes, then turn it out onto a wire rack and peel off the parchment. Wash the pan, dry it, and repeat with the orange, yellow, green, blue, and purple batters. Each layer bakes in about the same time, and you can reuse the two pans and the parchment if you re-grease them.
Step 4: Whip the Frosting
While the last few layers cool, beat 1 ½ cups softened butter in a stand mixer on medium speed until it looks fluffy and pale, about 3 minutes. With the mixer on low, slowly add 4 cups powdered sugar, scraping the bowl once or twice. Beat in ¼ cup sour cream, 2 tsp vanilla, and 1 tbsp milk. If the frosting is too stiff, add another tablespoon of milk. It should hold a peak but spread easily.
Step 5: Stack and Frost the Cake
Place the purple layer on a cake board or serving plate. Spread a thin, even layer of frosting over it, then top with the blue layer. Repeat with green, yellow, orange, and red, stacking in that order so the rainbow runs from purple on the bottom to red on the top.
Frost the top and sides with the remaining buttercream. A small offset spatula makes it easy, but the back of a butter knife works. Smooth the sides first, then swirl the top. The cake should be fully covered so no rainbow peeks through.
Step 6: Create the Unicorn Decorations
Dab a little frosting onto the sugar cone and roll it in the gold sprinkles until it is coated. Press it gently onto the top center of the cake, angled slightly forward.
Melt a few white chocolate wafers in the microwave in 20-second bursts, stirring until smooth. Spread it onto a piece of parchment into a thin rectangle about 3 inches wide and 4 inches long. Let it set for 5 minutes at room temperature, then use a sharp knife to cut two large triangles for ears. Press the ears into the frosting on either side of the horn. Toss a handful of rainbow sprinkles around the base of the horn and ears, and serve the cake right away or chill it for 30 minutes to set the frosting.
Tips for the Unicorn Cake
- Before you start baking, cut 6 parchment circles. It saves time and keeps the thin layers from sticking.
- If the layers dome up in the middle, press them gently with a flat spatula while they are still warm. They will flatten without crumbling.
- The buttercream must be truly room temperature. If your butter is cold, the frosting will look grainy and broken.
- For the cleanest slice, chill the cake for an hour and use a long, serrated knife to cut through.
Bake times by pan size:
| Pan size | Bake time at 350°F | Layers you will get |
|---|---|---|
| 8-inch round | 12 to 14 minutes | 6 thin rainbow layers |
| 9-inch round | 10 to 12 minutes | 6 slightly thinner layers |
| 6-inch round (x3) | 14 to 16 minutes | 6 layers, bake 2 at a time |
Keeping the Cake Fresh
- Refrigerate: Cover the cake loosely with plastic wrap or store it in a cake carrier. It stays moist for up to 4 days.
- Serve cold: I like it straight from the fridge, when the buttercream is firm and the cake is cool and dense.
- Reheating: This cake is not meant to be served warm. Let slices sit out for 15 minutes to take the chill off.
What to Serve with Unicorn Birthday Cake
A scoop of vanilla ice cream is obvious, but fresh strawberries are a better match. Their tartness cuts through the rich buttercream and brings out the vanilla in the cake. A simple bowl of whipped cream without sugar also works if you want something light, or serve a small dish of salted almonds on the side for a savory crunch that balances the sweetness.
Frequently Asked Questions
Can I make the cake layers a day ahead?
Yes. Wrap each cooled layer tightly in plastic and keep them at room temperature. Frost the cake the next day.
My layers came out different thicknesses. What went wrong?
The batter was probably not divided evenly. Use a kitchen scale to split the total batter by weight, then color each portion. You can also eyeball it with a cookie scoop.
How do I get the sugar cone horn to stay upright?
Press a small mound of frosting underneath it as glue, then chill the cake for 15 minutes so the frosting firms up.
Can I use liquid food coloring instead of gel?
You can, but you will need a lot to get bright colors, and it will thin the batter. Gel colors give more control.
This cake is so sweet. Can I reduce the sugar in the frosting?
The frosting sweetness is balanced by the sour cream and the salt in the cake. Do not cut the powdered sugar below 3 ½ cups or the frosting will be too soft to hold the layers.
Unicorn Birthday Cake Recipe
Ingredients
Method
- Preheat oven to 350°F. Grease two 8-inch pans and line with parchment.
- Whisk cake mix, buttermilk, melted butter, and eggs until smooth.
- Divide batter evenly into 6 bowls. Color each with one gel color.
- Spoon red batter into one pan, spread evenly, and bake 12 to 14 minutes. Cool 5 minutes, then turn out. Repeat for orange, yellow, green, blue, and purple.
- Beat softened butter until pale. On low, add powdered sugar, then sour cream, vanilla, and 1 tbsp milk. Adjust milk for spreadable consistency.
- Stack layers from purple to red, spreading frosting between each. Frost top and sides.
- Coat sugar cone with frosting and gold sprinkles. Melt white chocolate, spread thin, set, and cut into ear triangles.
- Press horn and ears onto cake. Scatter sprinkles. Chill 30 minutes before serving if you like a firm frosting.
