Preheat the oven to 350ºF. Prepare two 8 inch cake pans by lining the bottoms with parchment paper and generously greasing the sides and bottom of the pans with butter.

In a large mixing bowl, add the cake flour, baking powder, baking soda, and salt. Whisk together until completely combined. Set aside.

In a stand mixer, using the paddle attachment, cream the butter, peanut butter, and brown sugar. Mix on medium to high speed until the mixture is light and fluffy, about 3-5 minutes. Be sure to scrape the bowl down halfway through to prevent any lumps from sticking to the bottom of the mixing bowl.

Lower the mixer to slow speed and the egg yolks. Mix until the egg yolks are fully combined, about 1-2 minutes.

Add the vanilla. Mix until combined.

Add one third of the flour mixture and mix on low speed until just combined. Add half of the butter milk and mix. Add another third of the flour mixture, followed by the remaining half of the buttermilk, and finish off with the last of the flour mixture, mixing after each addition.

In a separate mixing bowl, whisk the egg whites and cream of tartar on high speed until stiff peaks begin to form, about 5 minutes.

Gently fold the egg whites into the batter until fully incorporated.

Pour the cake batter into the prepared cake pans. Bake for 28 minutes. At the 18 minute mark of baking, cover the cake pans with aluminum foil to prevent the tops of the cakes from burning. The cakes are done with a toothpick inserted comes out clean.

Remove the cakes from the oven and cool in the cake pans completely before flipping out. Once the cakes are cooled, frost and enjoy!