For the cake:
1¼ cups all-purpose flour
1 teaspoon baking soda
½ teaspoon salt
1 cup unsweetened cocoa powder, plus more for dusting the pans
2 teaspoons instant espresso or coffee powder
1 cup boiling water
½ cup sour cream
2 teaspoons vanilla extract
1 cup (2 sticks) unsalted butter, at room temperature
1¾ cups sugar
2 large eggs, at room temperature
For the frosting:
1½ cups plus 2 tablespoons (3 sticks plus 2 tablespoons) unsalted butter, at room temperature
3 cups confectioners’ sugar, sifted
Pinch of salt
1 vanilla bean, split lengthwise (optional)
2 tablespoons heavy cream
1 tablespoon vanilla extract
About 3 cups crushed Oreos, plus 8 whole Oreos for garnish
Heat the oven to 350˚. Butter the edges of 2 9-inch round cake pans and dust with cocoa powder, shaking out the excess. Line the bottoms of the pans with parchment paper.
In a medium bowl, whisk together the flour, baking soda and salt. In another medium bowl, whisk together the cocoa powder, espresso powder, and boiling water until smooth and set aside to cool slightly. Once cooled, whisk in the sour cream and vanilla and set aside.
In the bowl of an electric mixer, beat the butter on medium-high speed until smooth, about 1 minute. Gradually blend in the sugar and continue beating on medium-high speed until light and fluffy, 2-3 minutes more. Add the eggs one at a time, blending well after each addition.
With the mixer on low speed, add the dry ingredients in three additions alternating with the sour cream mixture, beating each addition just until incorporated. Divide the batter evenly between the prepared pans. Bake for 23-27 minutes or until a toothpick inserted in the center comes out clean, rotating the pans during halfway through to ensure even baking. Transfer the baked cake layers to a wire rack and let cool in the pans at least 30 minutes before inverting onto the rack to cool completely.
To make the frosting, add the butter to the bowl of an electric mixer fitted with the whisk attachment. Whip on medium-high speed until smooth, about 1 minute. Add the confectioners’ sugar to the bowl and mix on medium-low speed just until incorporated. Scrape the seeds from the vanilla beans into the bowl and add in the salt. Continue beating on medium-high speed until smooth, 1-2 minutes. Add the vanilla extract and heavy cream and beat on low speed just until incorporated. Increase the mixer speed to high and beat until light and fluffy, scraping down the bowl as needed, about 4 minutes.
To assemble the cake, place one of the cake layers on a cake board or serving platter. Spread an even layer of the frosting over the top of the cake, then top with the second cake layer. Frost the top and sides of the cake using a thin crumb coat, then place in the freezer for 30 minutes to set. Once the crumb coat is set, frost the top and sides of the cake again using an offset spatula. Cover the sides in crushed Oreos, and top the cake with whole Oreos, if desired.