Whisk together first 6 ingredients in top of a double boiler; add chocolate. Bring water to a boil. Reduce heat to medium-low; cook, whisking constantly, 24 minutes or until mixture reaches 160° (mixture will be moderately thickened).
Remove from heat. Spoon chocolate into demitasse cups, chocolate pots, or 4- or 6-ounce ramekins. Cover and chill at least 4 hours. Garnish, if desired.
Note: Demitasse cups or chocolate pots will yield smaller servings than ramekins, which is a good option for such a rich dessert.