Place the unpeeled potatoes in a large pot and cover with cold water. Add a generous pinch of salt. Bring to a boil, then reduce the heat and simmer for 20 to 25 minutes, until the potatoes are fork-tender.
Drain the potatoes. When cool enough to handle, peel them while still warm. Press the peeled potatoes through a potato ricer into a large bowl, or mash them until very smooth.
Allow the riced potatoes to cool completely.
If using bread cubes, melt the butter in a small skillet over medium heat. Add the bread cubes and toast for 3 to 4 minutes, stirring often, until golden. Set aside.
Add the potato starch, flour, beaten egg, salt, and nutmeg to the cooled potatoes. Mix gently just until a soft dough forms. Do not overmix.
Divide the dough into 12 equal portions. With lightly moistened hands, shape each portion into a smooth ball. If using bread cubes, flatten each portion, place one toasted cube in the center, then seal the potato dough around it.
Bring a large pot of salted water to a boil. Reduce the heat until the water is at a gentle simmer.
Carefully lower the dumplings into the water. Simmer gently for 12 to 15 minutes, until the dumplings rise and are cooked through. Do not allow the water to boil vigorously.
Remove the dumplings with a slotted spoon, drain well, and serve immediately with gravy, mushroom sauce, or melted butter and parsley.