Prick the potato all over with the tip of a knife or a fork, place it on a double layer of aluminum foil, and bake it 1 hour 45 minutes. Yes, that's a long time, but you want the potato to really caramelize. Juices will bubble from the pricks and cook to a dark caramel, and the flesh will be creamy and sweet.
Put the potato on a rack until it's cool enough to handle, then peel it and place the flesh in a small bowl. Scrape any caramelized flesh off the skin into the bowl, spoon in the Grand Marnier, and beat the potato with a fork until it's smooth.
Beat the heavy cream until it holds soft peaks, then cover it and refrigerate.
Put the sugar in a small deep saucepan (with a pouring spout, if you have one). Drizzle the syrup over it, then pour in 1/3 cup cold water. Use your finger to make an X in the sugar (this just makes sure it all gets moistened), then bring to a boil over medium-low heat. Once the syrup is boiling, increase the heat to medium, stick a candy thermometer in the pan, and start beating the egg whites in a large bowl with an electric mixer on medium speed.
By the time the whites have reached the soft-peak stage, the syrup should be at 240 degrees. Increase the speed to medium-high and carefully pour the syrup into the whites. Be careful not to burn yourself when you do this. Now increase the speed to high and continue beating until the bowl is cool, another 4 to 5 minutes. You'll have a very thick meringue.
Transfer the meringue to a larger bowl and fold in the sweet potato thoroughly. Give the cream a whisk or two, then fold it in. Spoon the mixture into a 3-quart aluminum souffle dish or into individual molds. Wrap first with plastic, then with aluminum foil, and freeze for at least 3 hours.
Serve with a pitcher of the chocolate sauce.
Bitter Chocolate Sauce
Bring the cream and corn syrup to a boil in a saucepan. Turn off the heat. Add the chocolate and shake the pan so all the chocolate is covered with cream. Let it sit for a few minutes to melt the chocolate, then stir until it's smooth. Add the bourbon and a pinch of salt. The sauce should be warm, so reheat it over simmering water if it cools down before dessert or if you've refrigerated it in the ubiquitous covered container.
Recipe courtesy One Potato, Two Potato (Houghton Mifflin, 2001) by Roy Finamore with Molly Stevens