Whisk together the flour, corn starch, salt, baking powder, and baking soda, until completely combined and set it to the side.
Next, cream together the butter and sugar in a stand mixer with the paddle attachment on medium-high speed until light and fluffy, which takes about 5 to 7 minutes.
Add in the vanilla to the mixer and then add in 1 egg white at a time until it is completely mixed in.
Pour in the oil. Stop and scrape the bowl.
Alternate adding in the dry ingredients with the buttermilk beginning with dry and ending with dry until mixed in on low speed. You should be adding the dry in 3 batches and the wet in 2 batches.
Fold in the sprinkles just until mixed in using a rubber spatula.
Distribute the batter evenly between 3 9” cake pans with parchment paper that have also been greased and floured. Bake at 350° for 27 to 29 minutes.
Cool completely on a rack, which takes about 90 minutes to 2 hours.
With about 15 minutes left in the cake cooling process, make the buttercream recipe.
Ice the cake in layers until it is completely coated.
Use extra icing in a piping bag with a star tip to make decorations on top of the cake. Also sprinkle with more sprinkles on top.
Notes
Make-Ahead: You can make this cake up to 1 day ahead of time.How to Store: Cover and keep at room temperature for 4-5 days. Cover and keep it in the refrigerator for 6-7 days.How to Serve: Funfetti cake is best served at room temperature, so I advise pulling it out of the refrigerator at least an hour before you plan to serve it.This recipe will convert to 24 muffins with a bake time of 22-25 minutes at 350°.You can substitute the buttermilk for whole milk. Add 1 tablespoon of vinegar to 1 cup of whole milk to make buttermilk.To make cake flour simply combine 2 tablespoons of cornstarch to 1 cup of all-purpose flour.