01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans, lining bottoms with parchment paper.
02 - In a medium bowl, whisk together flour, baking powder, and salt until well combined.
03 - Beat butter and sugar with an electric mixer on medium speed until light and fluffy, approximately 3 minutes.
04 - Add eggs one at a time, beating thoroughly after each addition. Mix in vanilla extract.
05 - Alternate adding flour mixture and milk to butter mixture, starting and ending with flour. Mix until just combined.
06 - Gently fold rainbow sprinkles into batter using a spatula, taking care not to overmix.
07 - Divide batter evenly between prepared pans. Bake for 25-30 minutes until a toothpick inserted in center comes out clean.
08 - Let layers cool in pans for 10 minutes, then transfer to wire racks to cool completely.
09 - Beat butter until creamy. Gradually add powdered sugar, milk, vanilla, and salt. Beat until light, fluffy, and spreadable.
10 - Frost top of one cooled layer, place second layer on top, and cover entire cake with remaining frosting. Decorate with additional sprinkles.