- Preheat the oven to 350°F (177°C). Grease two 9-inch round cake pans, line with parchment paper rounds, then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans. (If it’s helpful, see this parchment paper rounds for cakes video & post.)
- Whisk the cake flour, baking powder, baking soda, and salt together. Set aside.
- Using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter on high speed until smooth and creamy—about 1 minute. Add the sugar and beat on high speed for 2 minutes until creamed together. Scrape down the sides and up the bottom of the bowl with a silicone spatula as needed. Add the egg whites. (At this point, it’s best to use the whisk attachment.) Beat on high speed until combined, about 2 minutes. Then beat in the sour cream and vanilla extract. Scrape down the sides and bottom of the bowl as needed. Add the dry ingredients and mix on low speed until just incorporated. With the mixer still running on low, slowly pour in the milk until combined. Do not overmix. You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl. The batter will be slightly thick.
- Pour batter evenly into cake pans. Bake for around 24–25 minutes or until the cakes are baked through. To test for doneness, insert a toothpick into the center of the cake. If it comes out clean, it is done. Allow cakes to cool completely in the pans set on a wire rack. The cakes must be completely cool before frosting and assembling.
Source: https://sallysbakingaddiction.com/new-favorite-white-layer-cake/