Dry ingredients: Whisk together cake flour, baking soda, baking powder, and salt. Set that aside.
Wet ingredients: Bring out your handheld or electric mixer. Whip the unsalted butter until creamy (1 minute, high mixer speed). Add the white sugar and mix (3 minutes). Scrape the bowl thoroughly.
Beat in egg whites until combined (3 minutes), then mix in sour cream and vanilla. Scrap the bowl.
While the mixer is low speed, gradually add your dry ingredients (from Step 2) until JUST mixed in. Slowly pour whole milk into the running mixer. Don't overmix. Whisk with your hands (optional) to get rid of lumps.
Pour the batter into the cake pans. Bake them for about 25 minutes (so-so) until fully baked. To check, stick a small toothpick into the middle; it should be clean when you pull it out.
After baking time, let them cool in the cake pans on a wire rack.