Directions:
Preheat the oven to 350°F (177°C). Grease and lightly flour two 9-inch cake pans.
Whisk the cake flour, baking powder, baking soda, and salt together. Set aside.
Using a handheld or stand mixer fitted with a 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 rubber spatula as needed. Add the egg whites. Beat on high speed until combined, about 2 minutes. Then beat in the sour cream and vanilla extract. Scrape down the sides and up the bottom of the bowl as needed. With the mixer on low speed, add the dry ingredients 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.
Make the frosting: In a large bowl using a hand-held mixer or stand mixer fitted with a whisk or paddle attachment, beat the butter on medium speed until creamy – about 2 minutes. Add confectioners’ sugar, cream, vanilla extract, and salt with the mixer running on low. Increase to high speed and beat for 3 full minutes. Add more confectioners’ sugar if frosting is too thin, more cream if frosting is too thick, or a pinch more of salt if frosting is way too sweet.
Assemble and frost: First, using a large serrated knife, slice a thin layer off the tops of the cakes to create a flat surface. Discard (or crumble over ice cream!). Place 1 cake layer on your cake stand or serving plate. Evenly cover the top with frosting. Top with 2nd layer and spread the remaining frosting all over the top and sides. Decorate top of cake with sprinkles, if desired.
Slice, serve, enjoy!
Make ahead tip: The cake layers can be baked, cooled, and covered tightly at room temperature overnight. Likewise, the frosting can be prepared then covered and refrigerated overnight. Assemble and frost the cake the next day when you are ready to serve. Frosted cake can be frozen up to 2 months if you have room in the freezer. Thaw overnight in the refrigerator and bring to room temperature before serving.
Recipe Notes:
If you cannot get your hands on cake flour, you can make a DIY version from all-purpose flour and cornstarch. Measure 1 cup of all-purpose flour. Remove 2 Tablespoons. Mix in 2 Tablespoons of cornstarch. Sift it. Measure 1 cup. This sifted 1 cup equals 1 cup of cake flour. I suggest doing this 3x, then remove 1/2 cup since you need 2 and 1/2 cups in this recipe.
Whole milk and sour cream are strongly recommended for the best taste and texture. A full fat plain yogurt would work instead, though the cake may not be as light. Same goes with a lower fat milk.
To bake in a 9×13 pan: Simply pour the batter into a greased and lightly floured 9×13 pan and bake for 40 minutes or until a toothpick inserted in the center comes out clean.
From: https://sallysbakingaddiction.com/2017/01/02/new-favorite-white-layer-cake
