Chop the chocolate coarsely and set it aside.
In a heavy saucepan with a 1- to 2-quart capacity, add the coffee to the water and stir to dissolve. Add 1/2 cup (reserve 1/4 cup) of the sugar and place over moderate heat. Stir until the sugar is dissolved and the mixture barely begins to simmer. Reduce the heat to low, add the chocolate, and stir constantly until melted. Remove from the heat and set aside for a few moments.
In a large mixing bowl stir the egg yolks with a wire whisk just to mix. Gradually stir in about half of the warm chocolate, and then stir the yolks into the remaining chocolate. Stir in the vanilla or liquor, return to the large mixing bowl, and set aside.
In the small bowl of an electric mixer add the salt to the egg whites and beat until they barely hold a soft shape. Reduce the speed to moderate and gradually add the reserved 1/4 cup of sugar. Increase the speed to high again and beat briefly only until the whites hold a shape but not until they are stiff or dry.
One at a time, stir two or three tablespoonfuls of the whites into the chocolate. Then fold about half of the whites into the chocolate-do not be too thorough-and finally fold in the remaining whites.
The mousse may be poured into one large serving dish (it must have at least a 6-cup capacity but it may be larger), or gently pour it into a wide-mouthed pitcher and then pour into six to eight glasses with a 6- to 8-ounce capacity, or eight individual soufflé dishes. (Do not fill them all the way to the top.) Cover the serving dish or individual portions airtight with aluminum foil.
Refrigerate for about 24 hours.
In a chilled bowl with chilled beaters, whip the heavy cream, confectioners sugar, 1/2 tsp vanilla extract only until the cream holds a soft shape-not stiff. If you whip the cream ahead of time, refrigerate it. If so, it will probably separate while standing. Just stir/beat it a bit with a wire whisk before using.
Pour or spoon it over the mousse or pass it separately as a sauce.
Prep Time: 20 mins.
Cook Time: 10 mins.