Preheat oven to 325ºF. Place in the oven a baking pan with 1 inch of water in it to heat. In a separate bowl. Beat egg yolks with sugar until thick and creamy. Add very gradually to the cream, stirring constantly with a wooden spoon. Add vanilla extract and the 6 tablespoons Grand Marnier. Mix well.
Pour mixture into pot de crème cups and place in the baking pan of water in the oven. Bake for 35 minutes or until a knife inserted in the center of the custard comes out clean.
Remove from the oven and pan of water and cool completely. Chill in the refrigerator for at least 2 hours.
At serving time, blend grated orange rind with the 1 tablespoon Grand Marnier and add to the tops of the custards.