Bring two inches of water to a boil in a saucepan.

Place the chocolate and butter in a large bowl or bain marie that will fit 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 saucepan, and whisk in the egg yolks, one at a time. Add the liqueur. Set aside until cool to the touch.

Using an electric mixer or a whisk, beat the egg whites in a very clean bowl until foamy. Gradually add the sugar while continuing to beat. Continue whipping until the whites form shiny, soft peaks.

Place the heaving cream in a separate, large mixing bowl. Whip by hand or with an electric mixer until soft peaks form, being careful not to overwhip.

Gently fold the egg whites into the chocolate mixture until almost incorporated. Fold in the whipped cream until no streaks appear.

Place the mousse into individual serving dishes or one large serving bowl. Chill thoroughly, about 2 hours, and serve cold. Garnish with fresh fruit and/or chocolate shavings, if desired.