Make-ahead: the gnocchi can be boiled up to 4 hours ahead, and kept at room temperature.
1 1/2 pounds sweet potatoes, red skin variety
1/2 cup all-purpose flour, sifted, plus up to 1/2 cup for kneading
1/4 cup plus 2 tablespoons minced fresh sage
1/2 teaspoon salt
1 teaspoon freshly ground pepper
1/2 teaspoon nutmeg
1/2 cup grated Parmesan cheese, plus more for serving
1/2 cup (1 stick) unsalted butter
1. Preheat the oven to 350F/175C. Poke a few holes in each sweet potato to let steam escape, then place them on a baking sheet. Bake them until they're tender, about 45 minutes.
2. When the potatoes are cool enough to handle, peel them and pass them through a ricer, or a food mill with a fine grate.
3. Place the puree in a large bowl and add the flour, 2 tablespoons of the sage, the salt and pepper, nutmeg and the cheese. Mix gently until just combined.
4. Flour a work surface and scoop the dough out onto it. Dust the top and sides of the dough lightly with flour, and perhaps rub some flour between your hands so that they don't stick to the dough.
5. Divide the dough into 6 equal pieces. Roll each piece under your palms so that it forms a long rope. [The original recipe says that the ropes should be about 1 inch wide and 20 inches long, but your mileage might vary.] Go ahead and sprinkle the dough with a bit of extra flour if it's sticking. Cut the rope into 1-inch segments, and roll each segment over the tines of a fork to form the characteristic indentations. (Use a gnocchi board if you're lucky enough to have one.) Collect the formed gnocchi on a baking sheet.
6. When you've finished forming the gnocchi, bring a large pot of water to boiling and salt it generously. Drop several gnocchi into the boiling water (you'll need to work in batches so that you don't crowd the pot). Cook them for about 10 seconds after they rise to the surface. This could be only a couple of minutes, or up to five minutes. Use a slotted spoon to transfer the cooked gnocchi to a clean baking sheet, and cool completely.
7. Place a large, heavy saucepan over medium-high heat and add the butter. Melt the butter and cook it, swirling occasionally, until the milk solids have turned a lovely chestnut color and smell nutty, about 5 minutes. If you plan to add fried sage leaves as a garnish, go ahead and fry them.
8. Once the butter has browned, add the remaining chopped sage and turn off the heat. The mixture will bubble up, so don't be alarmed.
9. Add the gnocchi to the sage butter, place over medium-high heat, and cook until the gnocchi are heated through, about 5-6 minutes. You might have to do this in batches, in which case it's useful to have the oven preheated to 300F/150C so that your earlier batches don't get cold. Divide the gnocchi and sauce among bowls, garnish with sage leaves (if desired), and serve.