Spray 3, 8-inch round cake pans with cooking spray. Set them aside.
In a medium bowl, whisk together the flour, ¾ cup cocoa powder, baking powder, baking soda, and ½ teaspoon salt.
In a separate large bowl, combine the eggs, sugar, 2 teaspoons vanilla extract, sour cream, and vegetable oil.
Add the dry ingredients to the wet ones until well-combined, then slowly mix in the coffee.
Pour the batter evenly into the 3 prepared pans and bake for 35-40 minutes, until a toothpick inserted into the center of each cake comes out clean.
Let the cakes cool in the pans for 10 minutes, then remove them from the pans, and transfer them to a wire rack to cool completely.
To make the frosting, add the butter to a large mixing bowl and beat it with an electric mixer until it is smooth. Slowly add in the confectioner's sugar, espresso powder, remaining vanilla extract, salt, cocoa powder, and heavy cream. Mix on high until the frosting is creamy.
Add in an additional tablespoon of heavy cream if the frosting is still too thick.
To assemble the cake, pipe a layer of buttercream onto one of the cake layers, top with another cake layer, and pipe another layer of buttercream on top of that.
Add the third cake layer, then spread the remaining buttercream over the top and sides of the cake.