Preheat oven to 350°F (177°C). Grease three 6-inch 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 and sugar together on high speed until smooth and creamy, about 3 minutes. Scrape down the sides and up the bottom of the bowl with a silicone spatula as needed. Beat in the egg whites 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 just until combined. With the mixer still running on low, pour in the milk and mix just until combined. You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl. Gently fold in the sprinkles. 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.

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, heavy cream, vanilla extract, and salt with the mixer running on low. Increase to high speed and beat for 3 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 too sweet.

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 or, to replicate the decoration in the pictures and video, follow my directions in the recipe notes below. 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.