Preheat the Oven: Preheat your oven to 350°F (175°C). Grease and line three 8-inch round cake pans with parchment paper.

Prepare the Cake Batter: In a medium bowl, whisk together flour, baking powder, and salt. Set aside. In a separate bowl, beat butter and sugar together until light and fluffy, about 4-5 minutes. Add eggs, one at a time, mixing well after each addition. Stir in vanilla extract. Gradually add the dry ingredients, alternating with milk, beginning and ending with the dry ingredients. Mix until just combined.

Color the Batter: Divide the batter evenly into four bowls. Add a different gel food coloring to each bowl (one color per bowl). Gently fold the color into the batter until evenly mixed.

Bake the Cake Layers: Spoon the colored batter into the prepared pans, layering the colors in any order you like. Use a spatula to spread the batter evenly. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes before transferring them to wire racks to cool completely.

Prepare the Frosting: In a large bowl, beat the softened butter until creamy. Gradually add powdered sugar, one cup at a time, and continue to beat until smooth. Add vanilla extract and heavy cream, adjusting the amount of cream to achieve your desired consistency. Divide the frosting into separate bowls and color them with gel food coloring.

Assemble the Cake: Place one cooled cake layer on a cake stand. Spread a layer of frosting on top. Add the second layer and repeat. Top with the third layer of cake and apply a thin crumb coat to seal in the crumbs. Chill the cake for 20-30 minutes before applying a final thick layer of frosting. Use a spatula to smooth the sides and top.

Decorate: Decorate the cake with additional frosting, creating a rainbow of colors. You can use piping tips to create different designs or even pipe swirls of frosting around the edges. Add sprinkles, edible glitter, and a unicorn horn (if desired) for a magical touch.

Serve: Slice and serve your stunning Unicorn Cake at your next celebration!