Preheat your oven to 350°F (175°C).

Prepare your cake pans by greasing them with butter and lightly dusting with flour or lining them with parchment paper.

In a medium bowl, sift together the all-purpose flour, baking powder, and salt. Set aside.

In a large mixing bowl, beat the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.

Add the eggs one at a time, beating well after each addition.

Stir in the vanilla extract until fully incorporated.

Gradually add the dry ingredients to the butter mixture, alternating with the milk. Begin and end with the dry ingredients, mixing until just combined.

Divide the batter evenly into separate bowls, one for each rainbow color you desire.

Add gel food coloring to each bowl and mix until you achieve your preferred shades.

Pour each colored batter into the prepared cake pans.

Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cakes to cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

In a large bowl, beat the softened butter until creamy.

Gradually add powdered sugar, one cup at a time, beating well after each addition.

Mix in the vanilla extract and enough milk to reach a smooth, spreadable consistency.

Divide and color the frosting with pastel gel food colors as desired.

Level the cake layers if necessary to ensure they are flat.

Place the first layer on your cake stand or serving plate and spread a layer of buttercream frosting on top.

Add the next layer and repeat until all layers are stacked.

Apply a thin crumb coat of frosting all over the cake to seal in crumbs. Chill in the refrigerator for 15 minutes.

Apply a generous layer of frosting, smoothing it with a spatula for a flawless finish.

Sprinkle edible glitter and assorted sprinkles over the cake for a magical touch.

Attach the unicorn horn and ears to the top of the cake.

Add fresh flowers or fondant decorations as desired to enhance the whimsical appearance.