Melt the chocolate in a hot water bath and let it cool down a bit. Now separate the egg and beat the yolk with the sugar until white creamy. Stir in the chocolate and the orange liqueur. Separately, beat the egg whites and the cream until stiff.
Fold into the mixture and chill in the refrigerator for about 4 hours.
Peel and fillet the oranges. Now alternately layer the mousse and the orange fillets in four nice glasses.