Preheat oven to 325°F. Prepare a 9x13 inch baking pan by spraying with baking spray.

Toss the rainbow sprinkles with half a tablespoon of flour to prevent sinking, then set aside.

Mix remaining flour, baking powder, baking soda, and salt in a bowl and set aside.

Beat butter, sugar, oil, and vanilla until fluffy, about 3 minutes.

Add eggs one at a time, mixing well after each addition. Scrape down sides of bowl.

Alternately add dry ingredients and milk to the batter, starting and ending with dry ingredients. Mix until just combined.

Fold in sour cream and the sprinkles coated with flour.

Pour batter into prepared pan and bake for 40-45 minutes. Cool completely before frosting.

Make the frosting: Beat butter until smooth, then gradually add powdered sugar, vanilla, and cream until desired consistency. Add salt to taste.

Spread frosting over cooled cake and decorate with additional sprinkles if desired.