Method

• 01
  Place an oven rack in the middle of your oven and preheat to 350 degrees F. Grease, flour and line the bottoms of two 8-inch cake pans with circles of parchment paper.
• 02
  In a large bowl, sift together the flour, baking powder and salt. Set aside.
• 03
  In the bowl of a stand mixer fitted with the whisk attachment, whisk the egg whites until soft peaks form. Transfer the mixture to a bowl and set aside.
• 04
  Fit your stand mixer with the paddle attachment or a rotary beater. In the bowl (use the same bowl you just used for the egg whites; no need to wash), cream the butter until it's fluffy and light, 2-3 minutes on medium-high speed.
• 05
  Add the sugars and mix until combined and smooth, about 2 minutes on medium-high speed. Then add the egg yolks, one at a time, stirring after each and scraping the sides of the bowl with a rubber spatula to make sure everything gets incorporated. Stir in the vanilla.
• 06
  This is where we start to get wild: Add the sour cream and flour mixtures alternately, 2-3 additions each. Mix until the batter is smooth and lump-free. It'll be fairly thick.
• 07
  Fold the egg whites into the batter by hand, stirring gently just until no streaks of white remain.
• 08
  Divide the batter evenly between the two prepared cake pans. Bake for 30 to 40 minutes, rotating after the first 20 minutes. To see if your cakes are done, insert a tester or toothpick in the center of the cake and see if it comes out mostly clean.
• 09
  Remove the cakes from the oven, and let them cool for 5 minutes. Then run a sharp knife around the edges of each pan to loosen. Turn the cakes out onto wire racks, gently remove the parchment paper and let cool completely. Once they're cooled, level the cakes if needed.
• 10
  Time to make the buttercream! In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-high for 2 to 3 minutes, or until it's very smooth. Add 2 cups of the confectioners' sugar and the salt, stirring to combine.
• 11
  Start out on low, then move to a higher speed as the sugar gets absorbed. Continue adding the sugar, 1 cup at a time, until the buttercream reaches a spreading consistency you like. Then stir in the vanilla and any colorings you'd like to add.
• 12
  To assemble, stack one of the cake layers on a platter and apply a generous amount of buttercream on top. Spread that sweet stuff evenly all the way to the edges of the cake surface. Now place the second cake layer on top; if you've leveled the cake, place the cut side down so that the flattest surface is facing up.
• 13
  Apply a crumb coat — that's just a thin coat of buttercream that's meant to contain crumbs and doesn't have to look pretty. Chill in the refrigerator for 20 minutes.
• 14
  Once the crumb coat has set, use the remaining buttercream to ice the top and sides of the cake. Feel free to pipe designs and decorate to your heart's content, then garnish with sprinkles and birthday candles.