Preheat your oven to 350°F (180°C). Prepare two 9-inch cake pans (8-inch pans will also work) by greasing them with butter and lining the bottoms with parchment paper. For a perfect fit, trace and cut out parchment rounds to match the pan size.

In a medium bowl, sift together the flour, baking powder, baking soda, and salt. In a separate bowl, whisk the sour cream and milk until smooth. Set both bowls aside.

Using a stand mixer with the paddle attachment or a handheld mixer, cream the butter and sugar on medium speed until the mixture is light and fluffy, about 3-4 minutes. Scrape down the sides and bottom of the bowl as needed. Add the egg whites and vanilla extract, mixing until fully incorporated. Reduce the mixer speed to low and alternate adding the dry ingredients and the milk mixture, starting and ending with the dry ingredients. Mix just until combined, avoiding overmixing to ensure a light, airy cake.

Evenly divide the batter between the prepared pans and smooth the tops. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Let the cakes cool in the pans for 15 minutes before carefully removing them. Transfer to a wire rack to cool completely.