The perfect birthday cake!
Preheat oven to 350º F. Prepare three 8-inch round cake pans with nonstick baking spray and line the pan with a parchment paper round. (Don't skip this step)
In the bowl of a stand mixer fitted with the paddle attachment, cream together butter and shortening until light and fluffy, about 2 minutes. Stop and scrape sides as needed.
Slowly add sugar and make sure ingredients are fully incorporated.
Add eggs one at a time, making sure to fully incorporate each egg before adding another. Add in vanilla.
In a medium bowl, sift together flour, baking powder, and salt.
Add flour mixture to butter and shortening mixture alternately with milk mixture, beginning and ending with dry ingredients.
Remove bowl from stand mixer and stir in the rainbow jimmies by hand.
Evenly distribute cake batter between cake pans (roughly 2 1/4 cups per pan) and place pans into oven. Bake for 25-30 minutes, or until a toothpick or cake tester inserted in the center comes out mostly clean.
Cool in cake pans for about 10 minutes, then invert and cool completely on a wire rack.
These cakes have a beautiful crust on top which is delicious, but if you are stacking the layers you may want to level each layer first.
Whipped Buttercream Recipe (click link)