• Preheat the oven to 350° F and spray 2, 9” round cake pans or 3, 6” cake pans with baking spray and set aside.
• In a medium size bowl combine the 2 1/2 cup cake flour, 2 tsp baking powder, 1/4 tsp baking soda, and ½ tsp kosher salt. Whisk until combined.
• In the bowl of a hand mixer or stand mixer fitted with the paddle attachment cream the 1 cup butter for 1 minute on medium speed. Scrape down the bowl and add 1 ½ cups sugar.
• Mix on medium speed for 2 minutes until light and fluffy.
• Scrape down the bowl and add the eggs and yolk one at a time mixing in between. Add the 1 tbsp vanilla extract. Mix for 1 minute on medium speed.
• Add in 1/3rd of the dry ingredients, then half of the buttermilk and alternate, starting and ending with the dry ingredients.
• Mix well until combined. Do not overmix.
• Pour evenly into the cake pans. For 9” pans, bake for 30-40 minutes. For 6” pans check after 25-30 minutes. For 9x13, check after 35-40 minutes. The cake is done when the tops are firm and the sides are pulling away from the pan.

Make the chocolate buttercream

• When the cakes are cool, level the tops with a long serrated knife if they aren't straight enough.
• Place one cake on a cake board or serving tray. Spread chocolate frosting over to fill. Top with the other cake layer and frost all over. An offset spatula is the easiest thing to use but you can use a butter knife too.