For the cake:
- 3 eggs
- 2 egg yolks
- 1 tablespoon vanilla
- 3/4 cup sour cream
- 1/4 cup buttermilk
- 2 cups all-purpose flour
- 1 1/2 cups sugar
- 3/4 teaspoon baking powder
- 3/4 teaspoon baking soda
- 3/4 teaspoon salt
- 1/2 lb (2 sticks) unsalted butter, room temperature, cut into 1-inch pieces
For the frosting:
- 1 1/4 cups (2 1/2 sticks) unsalted butter, room temperature
- 3 1/2 cups powdered sugar
- 3/4 cup unsweetened cocoa powder
- 1/4 teaspoon salt
- 1 teaspoon vanilla
- 1/4 cup heavy cream or milk
- First, make the cake: Heat oven to 350 degrees F. Grease and flour two 8×2-inch round cake pans and line bottoms with parchment paper.
- In medium bowl, whisk eggs, egg yolks, vanilla, sour cream and buttermilk.
- In bowl of a stand mixer with paddle attachment, mix flour, sugar, baking powder, baking soda and salt on low speed until combined. While mixer is running on low speed, slowly add butter pieces one at a time until mixture looks like coarse sand. While mixer is running on low speed, add half the wet ingredients. Increase speed to medium until well mixed, about 30 seconds, then return to low speed. Add remaining wet ingredients, mixing until just combined. Increase speed to medium and beat 20 seconds. Scrape down sides of bowl with spatula and mix batter a few more times.
- Divide batter between prepared cake pans and smooth tops with spatula. Tap pans on counter twice to reduce air bubbles. Bake 17-22 minutes until cakes are golden brown and a toothpick inserted in center of each cake pan comes out clean.
- Cool cakes in pans on a cooling rack for 30 minutes, then turn cakes out onto rack, remove parchment paper, and cool completely before frosting.
- To make the frosting: In large bowl using electric hand mixer or in bowl of stand mixer with whisk attachment, beat butter on high speed 1 minute until smooth and creamy. Meanwhile, in medium bowl, whisk powdered sugar, cocoa powder and salt until combined. With mixer on low speed, slowly add dry ingredients. Add vanilla and cream. Once incorporated, increase speed to high and beat 3 minutes until well combined and fluffy. If frosting is too thin, add more powdered sugar. If frosting is too thick, add more cream.
- Frost cake, then top with sprinkles and candles and have a party.