Set 6 quarts of salted water to a simmer in a large pot.Mix mashed potatoes and egg yolk together.Add in flour and salt. If after the flour is stirred in the dough is still sticky, add more flour a little at a time until you have a fairly dry dough.Scoop out onto a lightly floured surface. Knead this a few times just to make sure everything is combined well.Divide dough into four even pieces and use your hands to roll one of the pieces into a long, even strand approximately ½? in diameter.Cut into individual dumplings an 1" long and use a fork to put the signature grooves in each dumpling which helps the gnocchi hold on to the sauce.Place gnocchi in simmering water and cook until they rise to the top which signals that they are done.Drain and top with sauce of your choice.