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.)

In a medium bowl, whisk the cake flour, baking powder, baking soda, and salt together. Set aside.

In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on medium-high speed until light and creamy, about 3 minutes. Scrape down the sides of the bowl with a silicone spatula as needed. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the egg whites and beat on high speed until combined, about 2 minutes. Scrape down the sides and bottom of the bowl as needed. Add the sour cream and vanilla extract, and beat on medium-high speed until combined, about 1 minute. Add the dry ingredients and then, with the mixer running on low speed, slowly pour in the milk and beat just 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 the batter evenly into the prepared pans.

Bake for 24–25 minutes or until a toothpick inserted in the center of the cake comes out clean. Cool the cakes in the pans set on a cooling rack for 1 hour. Run a knife around the edges to loosen the sides, remove the cakes from the pans, peel off the parchment, and place on the rack to finish cooling. The cakes must be completely cool before frosting and assembling.

In a large bowl using a handheld mixer or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Sift in the confectioners’ sugar and then add the cream and vanilla extract. Beat on low speed for 30 seconds, then gradually increase to medium-high speed and beat for 2 full minutes. Taste; beat in the salt if the frosting is too sweet. Add more confectioners’ sugar if frosting is too thin, more cream if frosting is too thick, or a pinch more salt if frosting is too sweet.

(For additional help with this step, see this how to assemble a layer cake video & post.) 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 one cake layer on your cake stand, cake turntable, or serving plate. Spread 3/4–1 cup (180–240g) of vanilla buttercream in an even layer on top. Top with the second cake layer, upside down, and spread the top and sides with 1 cup of buttercream (240g) in a very thin layer to make the crumb coat. If desired, run a bench scraper around the cake to help smooth out the frosting on the sides. Refrigerate the cake until the crumb coat has set, about 20 minutes. Finish by spreading the top and sides with the remaining buttercream. Decorate the cake with sprinkles, if desired.

Slice and serve immediately or keep at room temperature for a few hours before serving. Cover leftover cake tightly and store in the refrigerator for up to 5 days.