Melt chocolate in stainless steel bowl over hot water bath or top of double boiler. Add egg yolks and beat for 3 minutes. Remove from heat and add Grand Marnier. Allow mixture to cool.
Whip cream with sugar and vanilla until peaks are formed. Refrigerate cream for 10 minutes.
Meanwhile, beat egg whites until they form peaks.
Mix 2/3 of the chilled cream into the chocolate mixture, reserving the rest for garnish. Fold in egg whites. Chill mousse for 4-6 hours.
For the Grand Marnier crystallized sugar...
In a small sauce pan over low heat, mix 4 Tbl. sugar with 1 Tbl. water. Once combined increase heat to medium-high and bring to a boil. Stir mixture with a wooden spoon which will cause the sugar to crystallize.
Remove from heat, pour onto parchment paper and allow to cool, 2-3 hours.
Break crystallized sugar into small pieces, using fork to 'mash' into finer pieces. Soak in 2 Tbl. Grand Marnier.
To serve...
Divide crystallized sugar evenly between 4 dessert dishes. Fill dishes with chocolate mousse. Top with dollop of whip cream, a sprinkle of orange zest, and a fine grate of bittersweet chocolate.
Notes
* This is a sinfully sophisticated dessert that contains alcohol and is best suited for adults.