In a medium, heat-safe bowl, add the sugar, chocolate and butter and set over a medium saucepan of simmering water. Do not let the bowl touch the water or let the water come to a boil. Stir frequently until melted and smooth. Take off of the heat and whisk in the eggs, oil, espresso and vanilla. Whisk in the cocoa powder, baking soda and salt until well combined. Fold in the flour and set aside.