Bring an inch of water to a boil in a saucepan.

Place the chocolate and butter in a large bowl or bain marie that will fit on top of the saucepan without touching the water. Turn off the heat, and place the bowl on top of the saucepan. Allow the mixture to melt, stirring occasionally, until smooth. Remove the bowl from the pan.

Whisk in the egg yolks one at a time, and add the liqueur.

In a separate, clean bowl, beat the egg whites until they become foamy, using an electric mixer and clean beaters. Gradually add the sugar while continuing to beat. Keep whipping until the whites form shiny soft peaks.

Place the heavy cream in a third mixing bowl. Whip by hand or with an electric mixer until very soft peaks form (do not overwhip).

Gently fold the egg whites into chocolate mixture, one third at a time, until almost incorporated. Fold in the whipped cream until no streaks appear.

Place mousse into individual serving dishes or one large serving bowl. Chill thoroughly, about 2 hours. Serve cold.