Boil the potatoes in plenty of water until tender. Drain and peel while still hot. Sieve the potatoes onto a clean workbench or into a large bowl, using a potato ricer or food mill*. In a separate bowl, sprinkle salt over flour and mix well. Add beaten egg and half the flour to the potatoes. Knead, gradually adding more flour, until the mixture is soft, smooth and slightly sticky (depending on the potatoes, you might need a little more flour). Divide dough into quarters, shape into rolls about 2½ centimetres in diameter, then cut into two-centimetre pieces. Use a fork to make ridges in the gnocchi (optional). Bring five litres of water to the boil in a large saucepan. Do not put salt in the water, as this tends to make the gnocchi stick together. Meanwhile, heat butter in a large, flat ovenproof dish in a low-temperature oven. Drop the gnocchi into the boiling water, about 20 at a time. After they rise to the surface, cook for a further 20 seconds. Use a slotted spoon to remove from the pot. Dry on kitchen paper then transfer to the dish in the oven and toss lightly in butter. Repeat until all gnocchi are cooked.

* A food mill or potato ricer is necessary for this recipe. In their absence, push the potato through a sieve - but it's a bit more laborious.