01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans and line the bottoms with parchment paper.
02 - In a medium bowl, whisk together flour, baking powder, and salt until well blended.
03 - In a large bowl, beat butter and sugar with an electric mixer until light and fluffy, approximately 3 minutes.
04 - Add eggs one at a time, beating thoroughly after each addition. Stir in vanilla extract until incorporated.
05 - Alternately add flour mixture and milk to the butter mixture, beginning and ending with flour. Mix until just combined, being careful not to overmix.
06 - Gently fold in the rainbow sprinkles with a spatula until evenly distributed throughout the batter.
07 - Divide batter evenly between the prepared pans. Smooth the tops. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
08 - Cool layers in pans for 10 minutes, then turn out onto wire racks to cool completely before frosting.
09 - Beat butter until creamy. Gradually add powdered sugar, milk, vanilla, and salt. Beat until light, fluffy, and spreadable consistency is achieved.
10 - Once cake layers are completely cool, frost the top of one layer, place the second layer on top, and cover the entire cake with remaining frosting. Decorate with extra sprinkles as desired.