Bring sugar, water and grated orange rind to a boil over moderate heat. Cook syrup to 220 degrees on a candy termometer. Beat egg yolks until thick and lemon colored. Pour in syrup in a stream over egg yolks and beat until thick. Cool. Beat in Grand Marnier. Beat heavy cream until light shipped and fold into egg yolk mixture. Refrigerate.
Meanwhile, fit souffle dish with a 6" band of silver foil. Double and grease to form a collar extending 2" above rim and secure with a string. (Press foil closely along rim to prevent leakage.) Spoon a 2" layer of custard into dish and top with half the cake cubes. Cover sponge cake cubes with another layer of custard and add remaining cake cubes. Top with remaining custard. Freeze for 6 hours. Serve with whipped cream flavored with Grand Marnier. Garnish with strawberries and/or raspberries.