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 incorporated.
03 - Beat butter and sugar with electric mixer on high speed until light and fluffy, approximately 3 minutes.
04 - Add eggs one at a time, beating thoroughly after each addition. Mix in vanilla extract until combined.
05 - Add flour mixture in three additions, alternating with milk, beginning and ending with flour. Mix just until incorporated.
06 - Divide batter evenly between prepared pans. Bake for 30-35 minutes until toothpick inserted in center emerges clean.
07 - Let cakes rest in pans for 10 minutes before transferring to wire racks. Cool completely before frosting.
08 - Beat butter until creamy. Gradually incorporate powdered sugar until smooth. Add vanilla, salt, and milk to achieve spreadable consistency.
09 - Divide icing evenly among three bowls. Tint one purple, one green, and one gold using gel food coloring. Mix until colors are uniform.
10 - Position first cake layer on serving plate. Spread thin layer of any colored icing over top. Add second layer. Apply purple, green, and gold icing in alternating stripes or decorative swirls over sides and top.