Preheat oven to 350°F. Line four 7 inch pans (for taller layers) or 8 inch round pans with parchment rounds, and grease with non-stick cooking spray.
Mix together all dry ingredients (flour, baking powder, sugar, and salt) in a stand mixer with a paddle until fully combined.
Mix chunks of room-temperature butter slowly into the dry mix, on a low speed. Continue to mix until no large chunks of butter remain, and the mixture becomes crumbly.
Pour in egg whites, and mix on low until just incorporated. Mix in the buttermilk in two installments, on a low speed. Add in vanilla and oil, and mix at a low speed until fully incorporated. Scrape down the sides of the bowl with a spatula, then beat on medium speed for about 30 seconds.
If desired, add in gel food coloring. Mix the batter on a low speed, scrapping the sides and bottom of the bowl part way through. Mix until the batter is evenly colored.
Divide batter evenly between the prepared cake pans. I like to use a digital kitchen scale to weigh my pans, and ensure they all have the same amount of batter. This guarantees your layers will bake to be the same height.
Bake for 35-37 minutes if using 8 inch pans, or 37-38 minutes for 7 inch cake pans (or until a skewer comes out clean). Allow to cool for 10 minutes, then run an offset spatula around perimeter of the pan to separate the cake from the pan. Place cake layers into the freezer for 45 minutes, to accelerate the cooling process.
Once the layers have fully cooled, I sometimes like to trim the caramelized bits from the sides of the layers using a serrated knife. While these layers bake pretty flat, sometimes I’ll also trim the top of the layers too if I’m working a super special partnership ?