Preheat oven to 350°F (175°C). Line the bottoms of your cake pans with parchment paper and lightly grease the sides.

In a bowl, whisk together flour, baking powder, and salt.

In a mixing bowl, beat butter and sugar until light and fluffy. Add eggs one at a time, then mix in vanilla.

Gradually add the dry mix and milk, starting and ending with the dry ingredients, mixing just until no flour streaks remain.

Divide the batter evenly among small bowls and tint each with gel food coloring.

Layer colors into each cake pan by spooning dollops of batter around the pan, smoothing gently without mixing.

Bake for 22 to 28 minutes, until a toothpick inserted comes out clean. Cool in pans for 10 minutes, then remove to cooling racks.

Beat butter until smooth, then gradually add powdered sugar, mixing in vanilla, salt, and a splash of milk until creamy.

Level the cooled cake layers if necessary. Spread frosting between the layers and press gently to stabilize.

Apply a thin layer of frosting all over the cake (crumb coat) and chill for 15 to 20 minutes.

Frost the cake smoothly with a final coat of frosting, add marshmallow ears and a waffle cone horn, and pipe a colorful mane using a star tip.