Separate the yolks from the whites.
In a large bowl, beat the egg whites until stiff with an eggbeater or electric hand mixer. Set aside.
Without washing the beater, beat the egg yolks until light in color. Set aside.
Melt the chocolate, VERY CAREFULLY, in a glass bowl in the microwave for 20-second intervals, stirring after each interval, or in a double boiler over simmering water.
Remove the chocolate from the heat. Beat in the vanilla or liqueur, then beat in the egg yolks.
Add the chocolate mixture gradually to the beaten egg whites.
Fold until well blended.
Spoon the mousse into individual bowls, wine glasses, or a container, and refrigerate.