Preheat oven to 350°F/177°C. Butter and flour three 6 inch cake tins and line with parchment paper.
Combine all the dry ingredients (cake flour, salt, baking powder and baking soda) in a medium bowl, whisk together and set aside.
In a small bowl, mix the milk, sour cream and vanilla together. Set aside.
Using a handheld or stand mixer with the paddle attachment, beat the butter and sugar on medium speed (4) for about 3 minutes until smooth and creamy. Scrape down the sides of the bowl with a spatula.
Whisk the egg whites together with a fork and slowly drizzle the egg whites into the mixture. Keep it on low speed (2) until all the egg whites are added and then increase the speed to high (8) for about 1-2 minutes until combined. (May look curdled but it will smooth out as you beat). Scrape down the sides of the bowl.
On low speed, add half of the dry ingredients and mix until barely incorporated. Then add half of the milk mixture and mix for a few seconds. Then add the remaining dry ingredients mixing for a few seconds and then add the remaining milk mixture. Turn off the mixer and mix it by hand with a rubber spatula to make sure the batter is combined. Do not overmix the batter.
Pour the batter evenly into the pans (about a third of the way full). Bake for about 30-35 minutes until the cakes are cooked. Insert a toothpick into the center of the cake, if it comes out clean then it's done.
Allow cakes to cool completely (15-20 minutes) and then gently remove the cakes from the pans and allow them to rest on a wire rack.