Preheat the oven to 350 degrees F. Set out two 9-inch cake pans or one 9X13-inch baking dish. Grease and flour the pans. (If you plan to flip the cake out of the pan, line the bottom with parchment paper before greasing the pan. Trace the bottom of the pans onto parchment paper and cut the pieces out to fit exactly.)
In the bowl of a stand mixer, cream the butter and sugar on high speed, until ultra light and fluffy. (about 3-5 minutes) Then scrape the bowl with a rubber spatula.
In a separate bowl, mix the cake flour, baking powder, baking soda, and salt.
Pour the buttermilk in a large measuring pitcher. Then whisk in the eggs, egg white, and vanilla extract.
Turn the mixer on low and alternate adding in the flour mixture and the buttermilk, until all is combined. Scrape the bowl again. Then beat again for 15-30 seconds, just until the batter is smooth. Do not over-mix the batter.
Pour the cake batter in the prepared pan(s) and place on the center rack in the oven. Bake 9-inch pans for 22-25 minutes, or a 9X13-inch pan for 30-35 minutes. Test the cake by inserting a toothpick into the center. If it comes out clean, remove from the oven.
Cool the cake in the pan for 15 minutes, then carefully flip out onto a cooling rack. Cool completely. Once cooled, frost the cake with Homemade Funfetti Frosting. Then cut and serve!
Flour Measuring - It's very important to measure the flour properly. Stir the flour in the bag to lighten it up. Spoon the flour into measuring cups. Then level off the top.Learn how to make your own buttermilk here!What about cupcakes? Yes, you can scoop the batter into 24 cupcake liners. Bake for approximately 14-16 minutes.