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 for about 3 minutes until smooth and creamy. THIS STEP IS CRUCIAL for a fluffy cake!
Whisk the egg whites together with a fork and slowly drizzle them into the mixture. Keep it on low speed until all the egg whites are added and then increase the speed to high for about 1-2 minutes until combined.
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 or youll end up with a tough cake! Just fold gently until you don't see any more flour streaks.