Instructions: Preheat oven to 350F.

Place all the potatoes on a baking sheet. Bake until they are soft all the way to the center when pierced with the tines of a fork, about 1 hour for medium-sized Russets and 50 minutes for medium-sized sweet potatoes. Remove from the oven and let cool until they can be handled easily, about 15 minutes.

Peel the skins from the Russets, then, using the large holes of a hand-held grater, grate them into a large bowl. Peel the sweet potatoes and grate them into the bowl. Add the egg, and using an electric mixer set on medium speed, beat until fluffy, about 1 minute. Add the 1 cup flour, 1/2 teaspoon of the salt, and the parsley, and beat again just long enough to incorporate all the ingredients.

Fill a large pot three-quarters full with water and bring to a boil over high heat. While the water is heating, make the gnocchi: Lightly flour a work surface and turn the potato mixture onto it. Take a large handful of the dough and roll it first between the palms of your hands to make a rope about 8 inches long. Then, using your palms, roll it on the work surface into a rope 12 inches long and 1 inch in diameter. Cut the rope in half. Roll half the rope in the same manner to form another 12-inch-long rope, this one about 1/2 inch in diameter. Repeat with the other half of the original rope. Cut each rope into 1/2" long pieces. Lightly dust the pieces with flour and set them aside on a tray or platter. Cover them with a damp cloth until you are ready to cook them. Repeat this process, washing the stickiness from your hands if necessary, until all of the dough has been formed into gnocchi.

Preheat an oven to 250F. Place an ovenproof serving dish large enough to hold all the cooked gnocchi in the oven.

When the water is boiling and just before adding the gnocchi, add 1 teaspoon of the salt to the pot. Using a slotted spoon, slip the gnocchi into the boiling water 2 or 3 dozen at a time: do not crowd them in the pot. (You can cook the gnocchi all at once by using 2 large pots of water.) Cook the gnocchi 2 or 3 minutes, jogging them gently with a wooden spoon after a minute or two. They are done as soon as they float to the surface; cut into one to be sure. Remove them with a slotted spoon to the warmed dish and return it to the oven. Cook the remaining gnocchi in the same way.

When all the gnocchi are cooked, top them evenly with the butter, the remaining 1/2 teaspoon salt, the pepper and cheese. Increase the oven heat to 400F and return the serving dish to the oven. Cook the gnocchi for 8-10 minutes, or until the butter has melted. Serve piping hot.

NOTES : Similar in shape to Italian potato gnocchi, these pale golden dumplings have a smoother texture and the characteristic taste of sweet potatoes. Served with a roast and topped with its juices, they make a fine accompaniment, or offer them on their own, crowned with a little butter, grated Parmesan, and then gratineed, as here. Orange-fleshed sweet potatoes are not suitable for this recipe; choose instead starchy yellow-fleshed ones.

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