Preheat the oven to 350℉. Spray three 8-inch cake pans with baking nonstick spray. Line the bottoms with parchment paper circles. Spray again. Set aside.
In a medium bowl, sift the flour. Add in the baking powder, baking soda, salt, and dry milk powder.
Using a mixer in a large bowl, beat the butter, oil, and sugar on high speed for 2 minutes. It will become white in color and fluffy.
Add in the egg whites, vanilla bean paste, and sour cream. Mix on medium speed for 1 minute to add some air into the eggs.
Add in the dry ingredients and buttermilk. Mix on low speed until just combined. Take off the mixer and use a baking rubber spatula to scrape the bowl and make sure it is all mixed together.
Divide the cake batter evenly between the three cake pans. Bake for 25-28 minutes. Bake until a toothpick inserted comes out clean.
Let the cakes sit in the hot pans for 10 minutes. Then, transfer to a cooling rack and cool completely before decorating.