While the chocolate cools, whip the cream until it forms medium-stiff peaks. Do not overbeat or the mousse will lose some of its smooth, light texture. 3. Separate 4 of the eggs and set the whites aside. Combine yolks with the 2 remaining whole eggs in the bowl of an electric mixer. Beat until eggs are thick and lemon colored, around 5 minutes. While the yolks are beating, place the 4 egg whites in a clean copper or stainless steel bowl and whisk, preferably by hand, until the whites start to stiffen. Sprinkle on the confectioners' sugar and beat until you have firm peaks. 4. Working quickly, add the cooled melted chocolate and a scoop of the whipped cream to the egg yolks. Stir until smooth, then add the remaining cream. When it is fully incorporated, add the liqueur and vanilla, then fold in the whites until just blended. 5. This mousse can be poured into small individual serving dishes or into 1 larger dessert dish. Chill at least 4 hours before serving. (Chilling it overnight intensifies the flavor.) The mousse may be frozen for up to 2 weeks. Remove from the freezer and let it sit in the refrigerator overnight before serving. VARIATION: Make a Brownie Cookie Crust from the Brownie Cookies recipe, then fill it with the mousse. Chill for 4 hours, then pipe whipped cream on top.