Put chocolate, coffee, and sugar in a large metal mixing bowl. Set bowl over a large pot filled with 1 in. simmering water. Melt chocolate, stirring often, until smooth. Remove from heat and set aside.
Whisk egg whites in a medium bowl with a mixer until they hold stiff peaks. Whisk cream in another bowl until soft ribbons form.
Stir yolks into warm chocolate mixture. Mix in liqueur. Using a rubber spatula, gently fold in whites until almost blended, then gently fold in whipped cream until no streaks remain.
Pour chocolate mixture into a large (8-cup) glass serving bowl. Cover bowl tightly with plastic wrap and chill until ready to serve. Serve with sweetened whipped cream.
Really easy to make and excellent.
Note: The flavors of the coffee and Grand Marnier that the recipe calls for are really lost in the chocolate flavor of the mousse.
I added Grand Marnier with a bit of powdered sugar to the whipped cream topping and that added a nice flavor. A bit of orange zest on the whipped cream would probably be quite nice but I didn't try that yet.