Preheat oven to 350° and prepare cake pan with room temperature butter and cocoa powder. Coat the pan using about 2 tablespoons of butter using a pastry brush. Then dust the pan with 2 to 3 tablespoons of cocoa powder by tilting, tapping and turning the pan until evenly coated.
Bring water, butter, cocoa powder and espresso powder to a boil, whisking until combined. Turn off and remove from heat.
In a large mixing bow, whisk together flour, sugar, baking soda and salt.
In a separate mixing bowl mix together buttermilk (or sour cream), eggs and vanilla.
Mix the buttermilk and egg mixture into the dry ingredients then pour in the saucepan of warm chocolate mixture.
Whisk together until there are no lumps.
Pour the batter into the pan and bake for 45 minutes or until a toothpick comes out clean.
Allow the cake pan to cool on a wire rack for ten minutes. Then place a towel on the countertop and use another around the pan to hold it. Tap the pan on the countertop every few inches all the way around the pan. Place the wire rack on top of the pan and invert. If the cake doesn't release immediately, give it another few taps on the countertop until it releases. While the cake cools, prepare the icing.