Step 1

Place 3/4 cup sour cream, 1/2 cup buttermilk, and 2 large eggs in a medium bowl and let sit until room temperature. Meanwhile, arrange a rack in the middle of the oven and heat the oven to 350ºF. Coat 2 (9-inch) round cake pans with cooking spray or butter, then line the bottom of each with a parchment paper round.

Step 2

Add 1/2 cup vegetable or canola oil, 1 tablespoon vanilla extract, and 1 teaspoon kosher salt to the sour cream mixture and whisk until smooth.

Step 3

Place 1 3/4 cups all-purpose flour, 1 3/4 cups granulated sugar, 3/4 cup natural cocoa powder, 1 tablespoon instant espresso powder, 2 teaspoons baking soda, and 2 teaspoons baking powder in the bowl of a stand mixer (or large bowl if using an electric hand mixer). Beat with the whisk attachment on low speed until combined. With the mixer on low, add the sour cream mixture and beat until combined, about 1 minute 15 seconds. With the mixer still running, slowly pour in 1/2 cup boiling water and beat until just smooth, about 40 seconds.

Step 4

Divide the batter evenly between the pans (3 cups or about 21 ounces per pan). Bake until firm but a finger leaves an impression in the crust (a toothpick inserted into the center should still have a few crumbs), 25 to 30 minutes. Meanwhile, wash and dry the mixer bowl. Place 3 sticks unsalted butter in the bowl and let sit at room temperature until softened.

Step 5

When the cakes are ready, place on a wire rack and let cool completely in the pans, about 40 minutes. Meanwhile, make the frosting when the butter is ready.

Step 6

Beat the butter with the paddle attachment on medium speed until creamy, about 2 minutes. Add 4 1/2 cups powdered sugar, 1 cup natural cocoa powder, 1/4 cup heavy cream or whole milk, 1 1/2 teaspoons vanilla extract, and 1/4 teaspoon kosher salt. Beat on low speed for 30 seconds, then increase to high speed and beat for 1 minute. Do not over-whip. Beat in more cream or milk a tablespoon at a time as needed if the frosting is too thick.

Step 7

Run a knife around the edge of the cakes to loosen, remove the cakes from the pans, and discard the parchment. Place a dollop of frosting on the middle of a cake stand or platter and top with one of the cake layers. Dollop 1/3 of the frosting (about 1 1/2 cups) onto the cake and spread into an even layer, going all the way to the edges. Top with the second cake layer bottom facing up. Press down gently to adhere. Spread the remaining frosting onto the top and sides of the cake. Refrigerate for 30 minutes before slicing to help set the shape.