Preheat oven to 350°F. Grease three 8-inch round pans with parchment paper and cake strips (optional)
Sift flour, baking powder, and salt in a large bowl until smooth
Beat sugar, butter, and oil for 3 minutes until glossy
Add egg whites in three batches, mixing on low until incorporated after each addition
Combine buttermilk and vanilla, then mix into batter on low speed
Gradually add the dry ingredients to the wet ingredients in thirds, mixing on low
Fold in rainbow jimmies gently
Divide batter equally among prepared pans and bake 28-32 minutes until golden and toothpick-clean
Let cool completely before frosting
In a separate bowl, cream butter and add powdered sugar gradually with a hand mixer
Add salt, vanilla, and 3 tablespoons heavy cream; adjust texture with remaining cream
Frost cake layers and top with additional rainbow sprinkles