Preheat oven to 350F and grease three 9-inch cake pans.
First, mix together the dry ingredients. Whisk the cake flour, salt, baking powder, and baking soda and set aside.
Then, using a a handheld mixer, beat the butter and sugar together for about 3-4 minutes or until smooth and creamy. Use a rubber spatula to scrape down the sides of the bowl as needed. Beat in the eggs, egg whites, and vanilla extract until combined on high speed. Then, with the mixer on low speed, add the dry ingredients until just combined. Pour in the buttermilk and mix until just combined. Then, slowly fold in the rainbow sprinkles.
Pour the batter evenly into the three cake pans and bake for about 25 minutes or until golden. Check for doneness by inserting a toothpick into the center of each cake. Allow the cakes to completely cool before frosting and assembling.
While the cakes cool, make the frosting. In a large bowl, use a hand-held mixer and beat the butter for about 2 minutes on medium speed. Then, on low speed, add the confectioners’ sugar, vanilla extract, and salt.
Once the cakes are completely cool, assemble and decorate. This is best done on a cake turntable. Use an icing spatula to evenly cover each layer of cake and spread the remaining frosting over the top and sides. Refrigerate for 1 hour before slicing and serving.