Preheat the oven to 350?F (175?C). Grease and flour two 9-inch round cake pans.

In a large mixing bowl, whisk together flour, sugar, baking powder, and salt.

Add softened butter, eggs, vanilla extract, and milk to the dry ingredients. Beat with an electric mixer on medium speed for 2-3 minutes until smooth.

Gently fold in the rainbow sprinkles.

Divide the batter evenly between the prepared cake pans.

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

Allow the cakes to cool in the pans for 10 minutes, then transfer to a cooling rack to cool completely.

In a mixing bowl, beat the softened butter until creamy. Gradually add powdered sugar, vanilla extract, and milk, beating until smooth and fluffy.

Once the cakes are completely cooled, place one layer on a serving plate. Spread a layer of frosting on top.

Place the second cake layer on top and frost the top and sides of the cake.

Decorate the top with additional rainbow sprinkles.

Slice and serve the cake at room temperature.