Preheat oven to 350°F/180°C. Butter two 9-inch cake pans (8-inch would work too) that are at least 2-inch high, and line the bottoms with parchment paper (I like to cut out rounds of parchment the size of my cake pans).

In a medium bowl, sift together flour, baking powder, baking soda, and salt. In another medium bowl combine sour cream and milk. Set bowls aside.

In a standing mixer fitted with the paddle attachment or using a handheld mixer, beat butter and sugar on medium speed until light and fluffy, 3-4 minutes. Scrape down the sides and bottom of the bowl as needed. Add egg whites and vanilla extract and beat until combined. With the mixer on low speed, add half of the flour mixture and beat until combined. Add milk mixture and beat until combined. Add the other half of the flour mixture and beat just until combined. Do not over mix – the less you mix, the lighter the cake will be.

Divide batter evenly between prepared pans. Bake for 25-30 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.

Frosting: In the bowl of an electric mixer fitted with the paddle attachment, beat butter and salt on medium speed until smooth and creamy, about 2 minutes. Add the rest of the ingredients and beat on low speed for 30 seconds, then increase speed to high and beat for 2 minutes. Beat in more powdered sugar if frosting is too thin.

Assembly: If the cakes rose too much, cut their rounded top with a serrated knife to make them flat. Set one cake layer on a plate or cake stand. Evenly spread a thick layer of the frosting over the cake to the edge. Top with the second cake layer. Spread frosting over the top and sides of the cake. Frosted cake can be kept tightly covered in the fridge for 3-4 days. Leftovers can be kept in the freezer for up to 2 months and thawed overnight in the fridge.