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, about 3 minutes.
04 - Add eggs one at a time, beating well 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 with an offset spatula.
08 - Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
09 - Cool layers in pans for 10 minutes, then turn out onto wire racks to cool completely before frosting.
10 - Beat butter until creamy. Gradually add powdered sugar, milk, vanilla, and salt. Beat until light, fluffy, and spreadable.
11 - 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.