Preheat the oven to 325 degrees F. Toss the chocolate and butter into a small bowl and set over a small pot of simmering water to begin melting it. This will insulate the delicate chocolate from direct heat and keep it from scorching. As they begin to melt stir until smooth then remove them from the heat.

Whisk the eggs, sugar, vanilla and coffee together. Add the chocolate mixture, whisk until smooth then divide evenly between four coffee mugs.

Place the mugs in a small high-sided pot. Add enough hot water around the cups to equal the level of the batter within the cups. Bake until the custards are set, about 45 minutes. Check their doneness by jiggling them slightly. They should appear to be set but still have a bit of a quiver in the center. You may also insert a toothpick to judge doneness. They may take as long as 60 minutes but no longer.

Enjoy!