In a food processor, blend the ricotta, sugar and melted chocolate until smooth. Add the Grand Marnier and pulse until combined. Transfer the mousse to a large mixing bowl and set aside. In another mixing bowl, whip the cream to soft peaks. With a spatula, gently fold the cream into the ricotta mixture until smooth and combined. Spoon into glass dessert dishes and chill until ready to serve. Crisp cookies or tuile make a nice accompaniment. Can be made a day ahead and refrigerated.