Preheat your oven to 350°F (175°C). Prepare your cake pans by greasing and lining them with parchment paper.

In a bowl, mix together the flour, baking powder, and salt. In another bowl, beat butter and sugar until creamy. Mix in the eggs, milk, and vanilla until everything is well combined.

Gradually add the dry ingredients to the butter mixture, mixing until smooth. Pour the batter evenly into the pans.

Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Let the cakes cool for 10 minutes, then wrap in plastic wrap and freeze.

Beat butter until creamy. Gradually add powdered sugar, beating well after each addition.

Mix in vanilla and milk until the frosting is light and fluffy.

Divide the frosting into several bowls and mix in different food colors for decorating.

Place a layer of cake on a cake board or serving plate. Spread a layer of frosting on top and add another cake layer. Repeat until all layers are stacked.

Spread a thin layer of frosting over the whole cake to lock in the crumbs. Chill for 30 minutes.

Cover the cake with a generous layer of buttercream frosting. Use a piping bag to create a colorful mane, ears, eyes, and horn.

For the mane, pipe the colored frosting in rosettes or other designs to mimic a unicorn’s mane.

Attach pre-made or homemade decorations such as the unicorn horn and edible eyes.

Use additional colored frosting to add details and final touches, like the tail and facial features.

Chill the decorated cake for a short time to set the frosting.

Slice and serve the cake, enjoying the magical rainbow layers inside.