Prick the sweet potatoes and bake them directly on the oven rack at 400 degrees F for 50 to 60 minutes, until a knife slides in with no resistance. Do not wrap them in foil and do not boil them.

Split the potatoes open right away, scoop the flesh, and press it through a ricer onto a wide plate. Spread it thin and let it steam off for 10 minutes. You want about 2 cups.

Mix the cooled flesh with the ricotta, egg yolk, parmesan, salt and nutmeg.

Add flour a quarter cup at a time, folding lightly, and stop as soon as the dough holds together and no longer sticks to your palm. Most batches take about 1 ¾ cups. Do not knead it.

Cut the dough into four pieces, roll each into a rope about ¾ inch thick, and cut into 1 inch pillows. Roll each pillow down the back of a fork for ridges if you like. Lay them on a floured tray in a single layer.

Melt the butter over medium-low heat and cook the minced garlic for 2 minutes, until translucent with no color.

Stir in the flour and cook 1 minute, then whisk in the warm milk slowly and simmer 3 to 4 minutes until the sauce coats the back of a spoon.

Take the pan off the heat and whisk in the parmesan a handful at a time so it melts smooth. Season with black pepper and salt.

Boil the gnocchi in well salted water in batches of about 20. They float in 2 to 3 minutes. Give them 30 seconds more, then lift them out with a spider. Reserve a cup of the cooking water.

Add the gnocchi to the sauce over low heat and toss gently for a minute, loosening with a splash of the reserved water if needed.

Scatter the parsley over the top and serve with more grated parmesan.