Adjust oven rack to middle position and preheat oven to 350o degrees. Butter 2 8-inch square cake pans and line the bottoms with parchment paper. You can use round cake pans as well.
In a medium bowl, sift together the flour, baking powder, baking soda, and salt. Set aside.
In another medium bowl, mix the sour cream and milk. Set aside.
In a stand mixer fitted with paddle attachment, beat the butter and sugar on medium speed until light and fluffy - about 3-4 minutes - scraping the sides of the bowl as needed. Add the egg whites and vanilla extract and beat until combined.
With the mixer on low speed, add a third of the flour mixture and mix until just combined. Add half of the milk mixture - mix until combined. Add 1/3 of the flour mixture - mix until combined. Add the remainder of the milk mixture - mix until combined. Finish adding the remaining flour mixture and mix the final time by hand using the mixer paddle. Do not over mix each of these steps - the less you mix, the lighter and fluffier the cake will be. You will want to mix each step just until all of the ingredients are incorporated.
Divide the batter evenly between the prepared baking pans. Bake for 25-30 minutes until a toothpick inserted in the middle comes out clean.
Let the cakes cool in the baking pans for 15 minutes, then carefully remove from the baking pans and cool completely on a wire rack. If needed, gently run a knife around the edge of the cakes to loosen from the pans.