Preheat oven to 350°F. Spray three 6 inch pans with cooking spray and line bottom with parchment.
In the bowl of your stand mixer fitted with the paddle attachment, mix together flour, sugar, baking powder and salt until combined.
Cut butter into one inch cubes. With the mixer on low speed add the cubes of butter one at a time.
Add the egg whites and continue ue to mix on low until incorporated.
With the mixer still on low speed slowly pour in the buttermilk.
Add the vegetable oil and vanilla and mix until fully combined.
Stir in the rainbow jimmies.
Divide the batter evenly between the three prepared pans.
Bake for 30-35 minutes or until a toothpick comes out clean.
Let the cakes cool in the pans for about 10 minutes and then transfer to a wire rack to cool completly.