Preheat oven to 350°F / 175°C. Line four, 7-inch or three 8-inch pans with parchment rounds and grease with non-stick baking spray or homemade cake pan release. I used 7-inch cake pans.
Mix 3 cups cake flour, 3 cups sugar, 2 1/2 tsp baking powder, and 1 tsp salt together in a stand mixer with a paddle attachment or hand mixer until fully combined.
Mix in 1 cup of room temperature, unsalted butter slowly into the dry ingredients on a low speed. Continue to mix until no large chunks of butter remain, and the mixture looks like moist sand.
Pour in 1 cup of egg whites and mix on low until just incorporated. Scrape the sides and bottom of the bowl as needed with a rubber spatula.
Then mix in 1 1/2 cups buttermilk, 2 Tbsp of vegetable oil, and 1 tsp of vanilla extract on a low speed until fully incorporated. Scrape down the sides of the bowl with a rubber spatula one more time, then beat on a medium speed for about 30 seconds to make sure everything is properly mixed together. This also helps lighten the texture of the cake layers. The batter will be thin, but don't worry! That's how it's supposed to be.
If you make four 7-inch cake layers, pour half of the batter into a separate bowl (roughly 900g). Color one bowl light pink and the other bowl a deeper shade of pink using pink gel food coloring. Divide the batter evenly between the prepared pans, making 2 light pink and 2 deeper pink cake layers. I like to use a scale to make sure each pan has the same amount of batter and will bake up to the same height.
If you make three 8-inch cake layers, pour 1/3 of the batter into a separate bowl (roughly 600g). Color the bowl with more batter light pink, and the other bowl a deeper shade of pink using pink gel food coloring. Divide the batter evenly between the prepared pans to make 2 light pink cake layers and 1 deeper pink cake layer
Bake for 33-37 minutes or until a toothpick comes out with a few moist crumbs. Rotate the pans halfway through to help them bake evenly.
Let the pans cool for 10 minutes, then run a small offset spatula around the perimeter of the pan to separate the cake from the pan.
Place the cake layers into the freezer for 30 minutes to accelerate the cooling process. Once the layers are fully cooled, carefully flip the pans and remove the layers.
Use a serrated knife to level the tops and remove any caramelization around the sides of the layers. I also like to torte the cake layers horizontally (see picture above) to create thinner cake layers, but this is optional. You can also trim the layers, then wrap and freeze them if you're making them in advance.
If you make these cake layers in advance and freeze them, let them thaw for about 15 minutes before making the cake. The cake layers should still be slightly cold to the touch, which will make it easier to assemble the cake.