The cakes must be completely cool before frosting and assembling. Allow cakes to cool completely in the pans set on a wire rack. To test for doneness, insert a toothpick into the center of the cake. Bake for around 23-26 minutes or until the cakes are baked through. Weigh them to ensure accuracy, if desired.
You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl. With the mixer still running on low, pour in the buttermilk and mix just until combined. With the mixer on low speed, add the dry ingredients just until combined. (Mixture will look curdled as a result of the egg liquid and solid butter combining.) Scrape down the sides and up the bottom of the bowl as needed. Beat in the 3 eggs, 2 egg whites, and vanilla extract on high speed until combined, about 2 minutes. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. Parchment paper helps the cakes seamlessly release from the pans. Grease three 9-inch cake pans, line with parchment paper, then grease the parchment paper.
