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 thoroughly blended.
03 - Beat butter and sugar with electric mixer on medium-high speed for approximately 3 minutes until pale and fluffy.
04 - Add eggs individually, beating completely after each addition. Mix in vanilla extract.
05 - Alternate adding flour mixture and milk to butter mixture, starting and ending with dry ingredients. Mix until just combined.
06 - Gently fold rainbow sprinkles into batter using spatula. Avoid overmixing to prevent color bleeding.
07 - Distribute batter evenly between prepared pans. Bake for 25-30 minutes until toothpick inserted in center emerges clean.
08 - Let layers rest in pans for 10 minutes, then invert onto wire racks. Cool entirely before frosting.
09 - Beat butter until creamy. Gradually incorporate powdered sugar, milk, vanilla, and salt. Whip until light, fluffy, and spreadable.
10 - Frost top of one cooled layer. Position second layer atop. Cover entire cake with remaining buttercream. Press additional sprinkles onto sides and top.