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 well blended.
03 - Beat butter and granulated 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. Stir in vanilla extract until incorporated.
05 - Add flour mixture in three additions, alternating with milk in two additions, beginning and ending with flour. Mix just until combined, being careful not to overmix.
06 - Divide batter evenly between prepared pans. Smooth tops with spatula. Bake for 30-35 minutes until toothpick inserted in center comes out clean.
07 - Let cakes cool in pans for 10 minutes, then invert onto wire racks to cool completely before frosting.
08 - Beat butter until creamy in large bowl. Gradually add powdered sugar, mixing until smooth and creamy.
09 - Mix in vanilla and salt. Add milk 1 tablespoon at a time until spreadable consistency is achieved.
10 - Divide frosting equally among three bowls. Tint one bowl purple, one green, and one gold or yellow using gel food coloring. Mix each until color is uniform.
11 - Place first cake layer on serving platter. Spread thin layer of any colored frosting over top.
12 - Position second cake layer on top. Use offset spatula to frost cake with tricolor stripes or swirls, alternating purple, green, and gold frosting around sides and top.
13 - For enhanced presentation, sprinkle with colored sugars or edible glitter in Mardi Gras shades.