Line an 8-cup bowl with plastic wrap. Do not be concerned with the wrinkles. Melt the chocolate in the top of a double boiler over simmering water, stirring occasionally, until smooth. Remove the top part from the water. Whisk in the egg yolks and creme de cacao. In a large mixing bowl with an electric mixer on low speed, beat the egg whites until foamy. Increase the speed to high and beat until soft peaks form. Add the sugar, 1 tablespoon at a time, beating until stiff but still moist peaks form. Stir one-third of the whites into the chocolate mixture, then fold the chocolate into the rest of the whites until blended.

Spoon the mousse into the prepared bowl and freeze for about 1 hour, or until the mousse is firm enough to spread over the bottom and sides of the bowl, forming a thin shell. Return the bowl to the freezer until the chocolate is firm. Soften the vanilla ice cream slightly in the refrigerator and spread it over the chocolate mousse. Return the mold to the freezer until firm. Soften the peppermint ice cream slightly in the refrigerator and spread it in the center of the mold; smooth the top. Cover the ice cream with foil and freeze until solid. The ice cream ball may be frozen for several months, if desired.

Several hours before serving, invert the bowl onto a serving platter. Remove the bowl and the plastic wrap. Smooth the outside of the chocolate with a spatula. If desired, decorate the top by sprinkling it with crushed peppermint candy and decorate the bottom sides with candy canes. Return to the freezer until firm. Remove the mold from the freezer 20 to 30 minutes before serving to soften enough to cut into wedges.