Prepare the bread cubes (optional): Cut the stale roll into small cubes. Fry in butter over medium heat until golden and crispy. Set aside.
Cook and mash the potatoes: Peel the potatoes, cut into pieces, and boil in salted water until tender. Drain, let steam off briefly, then press through a potato ricer (or mash very finely) while still hot for a smooth base.
Make the dough: Season the hot riced potatoes with salt. Add 2–3 tbsp potato starch and 1–2 eggs, then mix quickly into a smooth, soft dough. The dough should be light and formable, not stiff. (If it feels loose, add ½ tbsp starch at a time until it just holds.)
Shape and cook the dumplings: With wet hands, form even dumplings (golf-ball to small-apple size). If you like, press 2–3 toasted bread cubes into the center of each and seal well. Bring a large pot of well-salted water to a gentle simmer (not a rolling boil). Slip in the dumplings and cook 15–20 minutes, until they rise to the surface and feel set yet delicate.
Serve: Serve immediately with roast meat, rich gravy, and red cabbage—the classic Thuringian combination.