1. Preheat the oven to 350 F and line a 6″ springform cake pan with parchment paper, and spray with non-stick spray. Dust lightly with flour.
2. In a large mixing bowl, whisk together the flour, cocoa powder, baking soda, baking powder and salt.
3. In a medium mixing bowl, whisk together the oil and sugar until combined. Whisk in the egg, instant espresso powder, and melted chocolate. Pour the wet ingredients into the dry, then add half of the buttermilk, stirring together. Add the remaining half and stir until thoroughly combined and there are no lumps. Scrape the batter into the prepared pan and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Do not under bake, as you will end up with a sunken center!
4. Allow the cake to completely cool before icing.*
5. Prepare the icing. In the bowl of a stand mixer fitted with the paddle attachment, or using an electric hand mixer, beat the cream cheese with the butter on medium-high speed until combined. Add half of the powdered sugar, along with the vanilla extract and mix. Add the remaining half of the powdered sugar and beat until just combined; the icing will be fairly stiff.
6. Ice the cake! If you will be serving the cake the same day, store it at room temperature, preferably covered. If serving in the next day or two, keep the cake (covered) in the refrigerator.

Leftover cake will keep at room temperature, in an airtight container, for 1-2 days, or in the fridge for 4-5. Cake can also be frozen prior to frosting. Wrap the cake (once cooled) tightly in plastic wrap and store for up to two months. Thaw overnight in the fridge before icing.