Preheat your oven to 350°F/175°C. Grease two 8-inch cake pans and line the bottoms with parchment paper.
In a medium bowl, sift together cake flour, baking powder, baking soda, and salt. Set aside.
In another clean and grease-free medium bowl, beat egg whites until stiff peaks. At this time, the whites have increased in volume and the peak holds its shape when you pull the beater/whisker out of the mixer. Gently fold in sour cream, half and half milk and vanilla extract using the rubber spatula (I usually do this step by hand) until combined. Set aside.
In a standing mixer fitted with the paddle attachment, beat butter and sugar on medium-high speed until light and fluffy, 3-4 minutes. Scrape down the sides and bottom of the bowl as needed to make sure all ingredients are incorporated.
With the mixer on low speed, add half of the dry mixture and beat until combined. Add wet mixture and beat until combined. Add the remaining half of the dry mixture and beat just until combined. Make sure not to over mix the batter, or you’ll end up with a dense cake.
Divide the batter evenly between the prepared pans and bake for 35-40 minutes until a toothpick inserted into the center of the cakes comes out clean.
Allow cakes to cool for 15 minutes, then gently remove from pan and allow them to cool completely on a wire rack.