Heat milk and butter to a boiling point. Mix egg yolks and sugar. Add flour. Mix again. Add a little hot milk mixture to egg yolk mixture. Cook and stir over medium heat until thickened but do not let boil. Remove from heat and add Grand Marnier. Beat egg whites until stiff but not dry. Fold into yolk-milk mixture slowly and carefully. Pour into buttered 2 litre souffle dish. Bake at 180ºC for 30-35 minutes and serve immediately.