In a small bowl, combine the oranges, sugar and Grand Marnier, to taste. Mix well. Whip the cream and sugar until stiff peaks. Flavor the cream with Grand Marnier to taste. To serve, place the bottom half of the shortcakes in the center of each serving plate. Spoon the oranges over the cake. Place the remaining cake on top of the oranges. Place some of the whipped cream on top of each shortcake. Garnish with mint and sugar.