Preheat the oven to 350°F and grease three 4-inch pans. Whisk the flour, baking powder, and salt. Beat the butter and sugar until fluffy, then add eggs and vanilla. Mix in the dry ingredients alternately with the milk until just combined. Divide among pans and bake for 15 to 18 minutes until a toothpick comes out clean. Cool completely.

Beat the butter until creamy, then gradually add powdered sugar. Mix in milk and vanilla and beat until fluffy. Divide among three bowls and tint with gel colors.

Trim the cake layers flat. Stack them with pink buttercream between the first two and blue between the next two, placing the final layer cut-side down.

Spread a thin crumb coat of buttercream over the whole cake and refrigerate for 20 minutes.

Fill a piping bag fitted with a star tip with the three colored buttercreams in stripes. Pipe a tall swirl on top and small rosettes around the edge. Decorate with gold sprinkles and a candy melt horn pressed into the top. Serve chilled or at room temperature.