2. Peel the sweet potato and cut it into chunks. Puree the sweet potato in a food processor with the egg yolks, brown sugar, orange zest, butter and salt until the mixture is smooth. If necessary, stop and scrape down the sides of the work bowl and continue pureeing.
3. Divide the mixture evenly among the 6 buttered ramekins. Tap firmly on the counter to settle the mixture evenly. (The recipe can be prepared ahead to this point and refrigerated tightly covered.)
4. Beat the egg whites until frothy. Add the cream of tartar and continue beating until stiff peaks form. The egg whites will be ready when they form well-defined pointed peaks that stand straight up and retain their shape.
5. Sprinkle the chopped hazelnuts over top of the egg whites and gently fold them in.
6. Divide the egg white mixture evenly among the ramekins, spooning them in a soft mound over the sweet potato mixture. Shake the ramekins gently from side to side to distribute the egg whites evenly.
7. Place the ramekins on a cookie sheet and bake at 375 F until the tops are puffed and golden brown in spots, 18 to 23 minutes. A sure sign of doneness is when the soufflé becomes extremely fragrant. Do not over-bake, or the center will be dry.
8. Remove and serve immediately.
Andrea Reusing is the creator of the restaurant Lantern in Chapel Hill, N.C., and author of the book Cooking in the Moment: A Year of Seasonal Recipes. In this installment of The Key 3, she shares with Lynne Rossetto Kasper the techniques behind three of her favorite recipes: Turnip Soup, Overnight Braised Short Ribs and Tomato Salad.