Prepare the Cake Batter

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

2. In a large bowl, cream butter and sugar with an electric mixer on medium speed until light and fluffy, about 3 minutes.

3. Add eggs one at a time, beating well after each addition. Mix in vanilla extract.

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

5. Alternate adding dry ingredients and milk to the butter mixture, starting and ending with dry ingredients (three additions of dry, two of milk). Mix until just combined, being careful not to overmix.

6. Divide batter equally into three bowls (about 2 cups per bowl). Tint each with a few drops of pink, blue, and purple gel food coloring, stirring until evenly colored.

Bake the Cake Layers

1. Pour each colored batter into a prepared cake pan, smoothing the tops with a spatula.

2. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.

3. Let cakes cool in pans for 10 minutes, then transfer to wire racks to cool completely.

Prepare the Buttercream

1. In a large bowl, beat butter with an electric mixer on medium speed until creamy, about 2 minutes.

2. Gradually add powdered sugar, 1 cup at a time, mixing on low speed until fully incorporated.

3. Add vanilla extract and heavy cream. Beat on high speed for 2-3 minutes until light and fluffy. Add more cream, 1 tablespoon at a time, if needed for spreadable consistency.

4. Divide buttercream into three equal portions. Tint each portion with pink, blue, and purple gel food coloring to match the cake layers.

Assemble the Cake

1. If necessary, level cooled cake layers with a serrated knife for even stacking.

2. Place the purple cake layer on a cake board or serving plate. Spread a 1/2-inch layer of purple buttercream on top.

3. Stack the blue layer and spread blue buttercream, then add the pink layer with pink buttercream.

4. Apply a thin layer of untinted buttercream (crumb coat) over the entire cake to seal in crumbs. Chill in the refrigerator for 30 minutes.

5. Frost the cake with a smooth layer of untinted buttercream, using a bench scraper for clean sides.

Decorate

1. Fill three piping bags with tinted buttercream (pink, blue, purple) fitted with star and round tips. Pipe swirls and rosettes on the top to create a rainbow mane.

2. Knead fondant until pliable. Shape a 4-inch cone for the horn and two small triangles for ears. Let dry for 1 hour.

3. Paint the fondant horn with gold edible paint and attach to the cake’s center with a dab of buttercream. Secure ears on either side.

4. Sprinkle edible glitter over the cake for a sparkling effect.