Separate the eggs. Add the sugar and Grand Marnier to the yolks and whisk briefly. Whisk the egg whites until stiff. Fold egg whites into yolk mixture.
Sprinkle with sugar and make criss-cross patterns on the top with a previously heated very hot poker. Serve at once.
This last process can be omitted if you feel you cannot do it quickly enough. Speed is very important or the soufflé will collapse.