In a stainless steel bowl set over a saucepan of boiling water, combine the egg yolks, Grand Marnier, sugar, and orange zest, and whisk to incorporate. Whisking constantly and removing from the heat from time to time, cook until the mixture has thickened and coats the back of a spoon into a sabayon, 4 to 5 minutes. Remove from the heat and cool.
Fold the whipped cream into the cooled sabayon, then fold in the melted chocolate. Refrigerate until well chilled, at least 6 hours. Transfer to a pastry bag fitted with a star tip.
To serve, place 1 tuile cup in the center of 8 dessert plates. Pipe (or alternately spoon) 1/4 cup of the mousse into the cups and garnish with the mint sprigs and powdered sugar. Serve.
Yield: 8 servings