If you are using the filling, melt the butter in a nonstick skillet over medium heat. Fry the bread cubes until golden brown on all sides, then set aside.

Boil the whole, unpeeled potatoes in lightly salted water until tender. Drain and let them cool just until you can handle them while they are still warm. Peel and pass them through a potato ricer or food mill into a large bowl.

Add the potato starch, salt, egg and nutmeg. Knead by hand until the dough is smooth and soft, without overmixing. It should not be sticky. If it is, add a little more potato starch.

Form 12 balls about 2 1/2 inches in diameter. For filled dumplings, press an indentation into each ball, add 2 to 3 croutons, close the dough around them, and roll back into a ball.

Bring a large pot of lightly salted water to a rapid boil, then reduce the heat. Lower each dumpling into the water on a spoon, holding it briefly before releasing it. Keep the water at a gentle simmer and cook uncovered for the full 20 minutes. The dumplings will float within a few minutes. Floating is not the cue to take them out.

Turn off the heat when the 20-minute simmer is complete. Serve with chopped parsley if you like.