On the day before you want to serve them, boil 1 1/2 lb potatoes in their jacket.
Peel the potatoes as soon as they are done, and press through a ricer or mash through a sieve; then cool them till the next day (not in the refrigerator, it will change the flavor) Next day peel and grate 1 lb potatoes.
Put grated potatoes in a cloth and wring out as much liquid as possible.
Add to the boiled potatoes.
Knead into this the egg, flour, and salt.
With floured hands shape into tennis ball sized balls.
In a large pot bring salted water to a full boil.
Put the dumplings into the water and boil 5 minutes, reduce heat to medium for and boil for 5 minutes more, reduce heat to low for and let simmer until done (dumplings will rise to the top when they are ready), about another 5 to 10 minutes.
Remove from water.
Caution, do not put a cover on the pot, the dumplings will disintegrate.