Preheat your oven to 325 degrees F. Spray three 8-inch or 6-inch round baking pans with nonstick spray, line the bottom of each pan with parchment paper and spray again. Set aside.
In a medium bowl or measuring cup, combine and stir the egg whites, whole egg and the vanilla. Set aside.
In a bowl of an electric mixer, fitted with a paddle attachment, combine the dry ingredients together on low speed for 30 seconds.
Add the butter one piece at a time, about every 10 seconds. Once all the butter is added, pour in the buttermilk and mix on low for about 1 minute, until the ingredients are incorporated.
Scrape down the sides of bowl and begin to add the egg mixture in 3 separate batches, mixing on medium-low until light and fluffy, about two minutes. Fold once or twice to ensure the batter at the bottom of the bowl is incorporated and mix for another 20 to 30 seconds.
Using a kitchen scale, divide batter evenly among your pans (about 16 ounces of batter in each of the 8-inch pans or 12 ounces in each of the 6-inch pans), spreading evenly with a small offset palette knife.
Bake until a cake tester or toothpick comes out with a few crumbs when inserted into the center, about 25 to 30 minutes. Check cake at 25 minutes and then set the timer for 2 to 3 minute intervals if the cake needs to bake longer. You're looking for a few moist crumbs to come out on the toothpick when inserted into the center of the cake.
Let the cake layers cool on racks for 10 minutes before inverting onto greased wire racks. Gently turn the cakes back up so the tops are up and cool completely.
Once the cakes are cooled completely, level the tops if needed.
Wrap each cake layer with plastic wrap and chill in freezer for at least an hour before using. The cake layers can be stored for up to a few days wrapped once in plastic wrap and frozen. To store longer than a few days, wrap twice in plastic wrap, then in foil, and seal in a zip-lock bag.