Please see notes section below for high altitude adjustments.
Preheat oven to 350°F. Grease two 9-inch round cake pans and line with parchment paper rounds and grease the paper. I like to use Pam for Baking for the spray.
Whisk together the cake flour, baking powder, baking soda, and salt in a large bowl and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or with a handheld mixer in a large bowl, beat the butter and sugar together on high speed until very light and fluffy (about 3 minutes). Scrab down the sides of the bowl.
Turn the mixer to low and add the egg yolks one at a time until fully incorporated. Beat in the vanilla extract.
Add about ⅓ of the dry ingredients and ½ of the buttermilk. Beat on low speed until combined. Add another ⅓ of the dry ingredients and the rest of the buttermilk and beat until combined. Finish with the dry ingredients and beat until just combined, but do not overmix. The batter will be thick.
In the meantime, whip the egg whites and cream of tartar in a small bowl with a handheld mixer on high speed until stiff peaks form (about 3 minutes). Gently fold the egg whites into the cake batter until incorporated. Do not overmix otherwise the egg whites will deflate!
Pour/spoon the batter evenly into the two cake pans and bake for 25 - 30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Cool cakes completely on a wire rack in the pan before assembling and frosting.