Pumpkin Gnocchi with Sage Butter

1 1/2 pounds fresh pie pumpkin, peeled and cut into 1-inch cubes
2 tablespoons butter
1 1/2 to 2 cups unbleached all-purpose flour, divided
1 large egg yolk
1 teaspoon salt
1/4 teaspoon freshly ground nutmeg, or to taste
1 stick unsalted butter
24 fresh sage leaves
2 ounces Parmesan cheese
Freshly ground black pepper, to taste

Pile the pumpkin cubes onto a rack over a cup of water in a medium-size saucepan. Steam over medium-high heat until the pumpkin is soft and tender, about 10 minutes. Lift the rack from the saucepan, discard the cooking water and allow the pumpkin to cool to room temperature. Force the steamed pumpkin through a food mill or press it through the holes of a colander. Put the purée back in the pan and stir in the 2 tablespoons of butter. Cook over low heat, stirring frequently, until the pumpkin has thickened and dried, about 10 minutes. Remove the pan from the heat and when the pumpkin purée has cooled almost to room temperature, stir in a cup of the flour, the egg yolk, salt, pepper and nutmeg. You should have a very sticky dough. Stir in another half cup of flour, then sprinkle the remaining flour onto a clean, dry surface and scrape the dough onto the flour. Knead the mixture gently, incorporating just enough flour to make a soft dough.

To form the gnocchi, divide the dough into 8 pieces. Roll each piece of dough into a rope about an inch thick and cut the rope into 12 pieces. Gently pinch each piece of dough with a well-floured thumb and forefinger to make a concave dumpling. Place the formed gnocchi in a single layer on a baking sheet lined with baker's parchment. If you're not going to cook them immediately, cover with plastic wrap and refrigerate the gnocchi for up to 4 hours or freeze them for several days.
When you're ready to cook the gnocchi, bring a gallon of water with a tablespoon of salt to a boil. Cook the gnocchi in two batches in the boiling water until they rise to the surface of the water, about 3 minutes. Lift the gnocchi out of the water with a slotted spoon and hold them on a warm platter. Toss the gnocchi with the butter sauce. Use a vegetable peeler to cut curls of Parmesan over the top of the dish and top with a few grinds of fresh pepper. To make the butter sauce, melt the butter in a small skillet over low heat. Add the sage leaves and simmer until the butter turns golden.