2. In a small saucepan, combine cornstarch, remaining 1/4 cup sugar and salt. Gradually whisk in milk to dissolve cornstarch. Cook, whisking often, over low heat, until thickened and simmering. Remove from heat. Add chocolate and butter. Let stand for 3 minutes, then whisk until smooth and melted. Whisk in egg yolk, orange zest, Grand Marnier, and vanilla. (The souffle can be prepared up to this point 2 hours ahead of time. Place a buttered round of waxed paper directly on the surface of the chocolate mixture and let stand in a warm place. The mixture must be warm, but not hot, before proceeding. Cook very gently over low heat, stirring constantly, if necessary, to reheat.)
3. In a medium, grease-free bowl, using a hand-held mixer set at medium-high speed, beat egg whites until stiff, but not dry. Stir about one-fourth of the whites into chocolate mixture to lighten. Transfer mixture into bowl of egg whites. Using a rubber spatula, fold until combined. Transfer to the prepared ramekins.
4. Bake until souffles are puffed and a toothpick inserted in center comes out clean, about 20 minutes. Remove from oven and let stand for 5 minutes. Run a knife around insides of ramekins to release. Invert onto two dessert plates and unmold. Sift confectioner's sugar over plates and cakes to dust completely. Arrange orange segments next to each cake. Serve warm.
