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1
Preheat an oven to 350 degrees Fahrenheit. Place the sweet potatoes on a rack positioned over a baking pan to allow circulation of heat, and roast until they are collapsed in appearance and soft inside when pierced with a knife.
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2
While the sweet potatoes cook, set up your work surface with the necessary ingredients and have extra flour on hand should you need it. Line a baking sheet with a clean kitchen towel.
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3
Allow the sweet potatoes to cool only enough to be able to handle them comfortably. They must be warm to form a successful dough. The flesh of the purple variety doesn't peel off easily, so best to scoop out the pulp with a spoon; discard the skin. If cooking the orange variety, peel off and discard the skin using a paring knife.
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4
Pass the sweet potato pulp through a ricer onto a floured work surface, forming a mound, or mash it finely using a potato masher or fork. Never put cooked potatoes in a food processor or blender.
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5
Scatter the orange zest, nutmeg, and sea salt over the potatoes. Sprinkle the mound with some flour and gradually work it in. Working quickly, keep adding the flour until you form a fairly smooth dough that no longer sticks to your hands. If necessary, add more flour to prevent stickiness. Scrape the work surface frequently as you work to keep it smooth and free of dried bits of dough as you work. You can sift any dried out bits of dough and flour back onto the board to keep the surface powdery and ease kneading. Once you have formed the dough, stop working it and cut it in half. Cover the remainder with an inverted bowl to keep it from drying out.
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6
Form the dough into ropes about 3/4 inch thick and as long as you like for ease of rolling. Use the dough scraper or a knife to cut it into cylinders as wide as they are thick. Use a butter paddle, the side of a box grater or a fork, take each little piece and dip it in flour on the cut ends to prevent sticking. Roll it against the paddle, grater or tines of a fork, pushing your thumb into it as you do so to form a hollow, concave dumpling with a pretty ridged surface. Place the gnocchi onto the prepared towel-lined baking sheet. Repeat this process with the remaining dough to form the remaining gnocchi, lining them up on the towel without touching.
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7
Have ready a spider strainer or a slotted spoon with which to scoop the cooked gnocchi out of the cooking water. Fill a pot with 5 quarts water. Select a shallow serving platter and spread the butter in it. Bring the water to a rapid boil and add 2 tablespoons kosher salt.
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8
Lift the towel with two corners in the grasp of each hand, and position it over the boiling pot. Release your hold of the bottom two corners of the towel and drop the gnocchi at once into the water. Cook over high heat until the dumplings float to the top, allowing them to bob on the surface no longer than 1 minute before you retrieve them with the spider strainer or slotted spoon.
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9
Transfer the gnocchi to a warm, buttered serving platter, shaking the dish to toss and coat them all over. Scatter with the hazelnuts and grated cheese and serve at once.
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10
Note: Once formed, the gnocchi can be left out at room temperature, uncovered, for up to two hours, or frozen in place in an ample deep-freeze. Once frozen solid, slide the gnocchi into freezer bags and freeze for up to three months. To cook, drop them, frozen into boiling salted water and proceed as described in the recipe
