Preheat oven to 350°F. Grese the inside of your cake pans with butter and then dust with flour- or spray with a floured non stick baking spray.
Melt the 10 tbs butter in the microwave then set aside and let it cool slightly.
Combine flour, baking powder, baking soda, salt, and 1 ½ cups sugar in a large bowl, set aside. (2 1/2 cups cake flour, 1 1/4 tsp baking powder, 1/4 tsp baking soda, 3/4 tsp salt, 1 1/2 cups sugar)
Whisk buttermilk, melted butter, oil, vanilla and egg yolks until well combined, set aside. (1 cup buttermilk, melted 10 tbs butter, 3 tbs vegetable oil, 2 tsp vanilla, 6 large egg yolks)
Using a stand mixer or electric hand mixer and the whisk attachment, whisk egg whites and cream of tartar on medium speed until foam appears (about 1 minute). Then whisk on high until the egg whites become cloud like (1-2 minutes). (3 large egg whites, pinch of cream of tarter)
Slowly add remaining ¼ cup of sugar to the egg whites and whisk until stiff peaks form (3-5 minutes). Move egg whites to a different bowl.
In the stand mixer bowl with the paddle attachment add the flour mixture. While whisking on low speed, slowly add the buttermilk mixture, then increase the speed to medium high and make sure it is fully incorporated.
Using a large spoon or rubber spatula fold in, by hand, 1/3 of the egg white mixture into the batter. Then carefully fold in the remaining egg white mixture making sure there aren’t any white streaks when done.
10. Pour the batter into the prepared cake pans. Bake for 20-25 minutes, or until a toothpick inserted comes out clean. If baking more than one cake layer at a time, bake the cakes on the same oven rack to ensure even baking.
11. Let the cakes cool completely before frosting.