Preheat the oven to 350°F (175°C).

Grease and flour three 8-inch round cake pans.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

In a large bowl, cream together the butter and sugar until light and fluffy.

Add the eggs, one at a time, beating well after each addition.

Stir in the vanilla extract.

Gradually add the dry ingredients, alternating with the buttermilk and milk, beginning and ending with the dry ingredients.

Mix until just combined. Gently fold in the rainbow sprinkles.

Divide the batter evenly among the prepared cake pans and smooth the tops.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Let the cakes cool in the pans for 10 minutes before transferring them to a wire rack to cool completely. For the frosting, beat the butter until creamy.

Gradually add the powdered sugar, 1 cup at a time, beating well after each addition.

Add the vanilla extract, heavy cream, and salt, and continue to beat until smooth and fluffy.

Once the cakes have cooled, place one cake layer on a serving plate.

Spread a layer of frosting over the top and repeat with the remaining cake layers.

Frost the entire cake with the remaining frosting.

Garnish the top of the cake with extra rainbow sprinkles.