01 - Prick the sweet potatoes with a fork and place them on a baking sheet. Bake at 400°F for 40 to 50 minutes, or until very tender. Let cool slightly, then peel and mash until completely smooth.
02 - Transfer 1 1/2 cups of mashed sweet potato to a large mixing bowl. Stir in the egg, salt, and nutmeg. Gradually add the flour, mixing to form a soft, slightly sticky dough.
03 - Turn the dough onto a floured surface and gently knead, adding more flour only if needed, until smooth and pliable. Divide dough into 4 pieces, then roll each into a rope about 3/4 inch thick. Cut into 3/4-inch pieces. Roll each piece over a fork to create ridges, if desired.
04 - Bring a large pot of salted water to a boil. Working in batches, add gnocchi and cook until they float to the surface, about 2 to 3 minutes. Remove using a slotted spoon and set aside.
05 - In a large skillet, melt butter over medium heat. Add sage leaves and cook until the butter is golden and aromatic, 2 to 3 minutes.
06 - Add cooked gnocchi to the sage butter and gently toss to coat. Season with salt and pepper to taste.
07 - Serve immediately, topped generously with freshly grated Parmesan cheese.