Preheat oven to 350 degrees F. Grease two 9-inch round cake pans with butter and flour or cooking spray and line the bottoms with parchment paper. For best results, use bake-even strips. (see note)
In a large bowl or stand mixer fitted with a whisk attachment, whisk together the sugar, flour, cocoa powder, baking soda, baking powder, and salt.
Beat in the eggs, buttermilk, espresso, oil, and vanilla to the dry ingredients. Beat on medium speed for 2 minutes, the batter is going to be very thin.
Divide the batter evenly between the two prepared 9-inch pans.
Bake the cakes for 40 to 45 minutes until a toothpick inserted in the center comes out clean.
While the cakes are baking, begin preparing your frosting since it will also need to cool for 1 hour.
Cool the cakes in pans for at least 10 minutes. Then loosen from the sides of the pans before transferring to a cooling rack to cool completely, about 1 hour. (If your baked cakes have a domed top, use a serrated knife or cake cutter to cut the domed section and create a flat top.)
After the cakes have cool and the frosting has been prepared, place one of the cake layers on a cake stand or serving plate.
Pipe or spread a thick layer of the frosting all over the top of the cake layer to create the filling layer.
Top the filling with the other cake layer and frosting the entire cake with a thin layer of frosting. (This creates a crumb coat and keeps any crumbs from getting into the frosting on the final coating.)
Chill the cake for 30 minutes.
Once the crumb coat is set, frost the top and sides of the cake with the remaining frosting. You can pipe decorative designs or even use the back of a spoon to create swirls on top.
Notes
If you don't have buttermilk on hand, you can substitute sour milk. Combine 1 cup of whole or 2% milk with 1 tablespoon of white vinegar and stir. Let sit for 5 minutes before using. This cake can be made in advance. The fully frosted cake can be stored in the refrigerator or at room temperature for 1 day uncut.The slices of cake will remain good at room temperature in an airtight container for up to 3 days.The cakes themselves (unfrosted) can be frozen. Let them cool completely and then wrap in plastic wrap and place them in a freezer bag, squeezing out any excess air and freeze for up to 3 months. Thaw in the fridge overnight before frosting.You can purchase bake even strips or you can use this homemade method which I used since my strips were otherwise engaged.