Peel potatoes and boil in salted water until tender.
Drain and mash while steaming hot. Add half the butter, salt, pepper, nutmeg, cream and mix well
Taste for salt. When mixture becomes cooler, add the egg yolks and the parmesan cheese and mix again.
Grease the moulds with remaining butter, and sprinkle each with bread crumbs.
Fill moulds halfway with prepared mixture.
Next, layer a generous piece of mozzarella in each, then fill moulds to the top with remaining mixture.
Level off the top surface of each, and cover with a thin layer of bread crumbs.
Bake for about 25 minutes at 325°F (170°C).