Special equipment: 1-liter cream whipper and 1 N2O (nitrous-oxide) charger
Create an ice bath by filling a large bowl with ice and water.
Add enough water to a 10-inch straight-sided saute pan to come up the side 1 inch. Bring to a simmer over medium heat.
Combine the chocolate, coffee, water and sugar in a medium metal bowl and set in the simmering water. Stir occasionally until the chocolate is almost completely melted and the sugar is dissolved, 4 to 5 minutes. Remove from the heat and set the bowl in the ice bath. Whisk until the mixture reaches 60 degrees F and is the consistency of heavy cream, 2 to 3 minutes. Pour the mixture into the cream whipper. Do not fill above the maximum fill line.
Charge the cream whipper with 1 charger. Shake vertically 16 to 20 times. Set aside for 1 minute. Invert and dispense into small bowls or martini glasses. Serve immediately.