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.

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.

Add the egg whites and beat on high speed until combined, about 2 minutes.

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.

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 pan, peel off the parchment, and place on the rack to finish cooling.

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.

Assemble and frost: Slice a thin layer off the tops of the cakes to create a flat surface. Place one cake layer on your cake stand, spread ¾ to 1 cup 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 in a very thin layer to make the crumb coat.

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.