Break the chocolate into large pieces and melt in a hot water bath. Let it cool down a bit, but it still has to stay supple. Separate the eggs, beat the egg whites into egg whites and refrigerate.
Mix the egg yolks with the sugar until frothy. Whip the cream until stiff while drizzling in the vanilla sugar. Now stir the egg yolk mixture into the chocolate, pour in the Grand Marnier, fold in the cream and then fold in the egg whites.
Put the mousse in the fridge for a couple of hours to set.