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Preheat oven to 350°F (177°C). Grease three 6×2 inch cake pans, line with parchment paper, then grease the parchment paper. Parchment paper helps the small cakes seamlessly release from the pans.
Whisk the cake flour, baking powder, baking soda, and salt together. Set aside.
Using a handheld or stand mixer fitted with a paddle 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 and vanilla extract. Beat on medium-high speed until combined, then beat in the sour cream. 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 over-mix. 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 18-21 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’s done. Allow cakes to cool completely in the pans set on a wire rack. The cakes must be completely cool before frosting and assembling
With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Add 4 and 1/2 cups confectioners’ sugar, the heavy cream, and vanilla extract. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 2 full minutes. Add up to 1/2 cup more confectioners’ sugar if frosting is too thin or another Tablespoon of cream if frosting is too thick. Add a pinch of salt if frosting is too sweet. (I add 1/8 teaspoon salt.)
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, cake turntable, or serving plate. Evenly cover the top with about 3/4 cup of frosting. Top with 2nd cake layer and evenly cover the top with about 3/4 cup of frosting. Top with the third cake layer. Spread the remaining frosting all over the top and sides. A bench scraper and small offset spatula are handy for decorating. Decorate with sprinkles if desired.
Refrigerate for at least 30-45 minutes before slicing. This helps the cake hold its shape when cutting.
Cover leftover cake tightly and store in the refrigerator for 5 days.