Preheat oven to 350°F (175°C). Grease and line your cake pans.

In a large bowl, cream butter and sugar until light and fluffy. Add eggs one at a time, mixing well after each addition.

Mix in vanilla extract. Alternate adding flour and milk, starting and ending with flour. Mix until just combined.

Divide the batter into separate bowls. Add gel food coloring to each bowl for rainbow layers.

Pour each colored batter into prepared cake pans. Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean. Cool completely.

In a mixing bowl, beat butter until smooth and creamy. Gradually add powdered sugar, beating on low speed.

Add vanilla extract and heavy cream. Beat on high until frosting reaches a fluffy consistency.

Divide frosting into bowls and add gel food coloring to create rainbow shades.

Level the cooled cake layers if needed. Stack them with a thin layer of frosting between each layer.

Apply a crumb coat to the entire cake and chill for 30 minutes.

Apply a smooth layer of frosting using an offset spatula.

Pipe the rainbow mane with different colored buttercream using various piping tips.

Sculpt a unicorn horn and ears from fondant or modeling chocolate. Paint the horn with metallic edible paint.

Apply edible glitter to the horn and mane.

Sprinkle pastel-colored toppings along the base or border of the cake.

Chill the cake until ready to serve.