01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans and line 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 electric mixer on medium-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.
05 - Alternate adding flour mixture and milk to butter mixture, starting and ending with flour. Mix until just incorporated, being careful not to overmix.
06 - Gently fold rainbow sprinkles into batter using spatula. Distribute evenly without overmixing to prevent color bleeding.
07 - Divide batter evenly between prepared pans. Smooth tops. Bake 25-30 minutes until toothpick inserted in center emerges clean.
08 - Let layers cool in pans 10 minutes. Remove from pans and transfer to wire racks to cool completely before frosting.
09 - Beat butter until creamy. Gradually add powdered sugar, milk, vanilla, and salt. Continue beating until light, fluffy, and spreadable consistency achieved.
10 - Once layers completely cool, spread frosting on top of one layer. Stack second layer on top. Cover entire cake surface with remaining frosting. Decorate with additional sprinkles.