01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans, lining bottoms with parchment paper.
02 - Whisk together flour, baking powder, and salt in a medium bowl until evenly blended.
03 - Beat butter and sugar with electric mixer on medium-high speed until light and fluffy, approximately 3 minutes.
04 - Add eggs one at a time, mixing thoroughly after each addition. Blend in vanilla extract.
05 - Add flour mixture in three additions, alternating with milk in two additions, beginning and ending with flour. Mix only until combined.
06 - Divide batter evenly between prepared pans. Bake 30–35 minutes until wooden pick inserted in center emerges clean.
07 - Let cakes rest in pans 10 minutes before inverting onto wire racks. Cool completely before frosting.
08 - Beat butter until creamy. Gradually incorporate powdered sugar, mixing until smooth. Add vanilla and salt. Blend in milk 1 tablespoon at a time until spreadable.
09 - Divide icing among three bowls. Tint one portion purple, one green, and one gold using gel food coloring. Mix each until uniform color achieved.
10 - Place first cake layer on serving platter. Spread thin layer of any colored frosting. Top with second layer. Apply tricolor frosting in stripes or swirls around sides and top, alternating colors for festive pattern.