Whip egg yolks and salt together with a fork. Strain through a sieve to remove hard white particles that adhere to yolks.
Heat stock to a simmer.
Holding your finger over the opening, fill a very fine funnel with the yolk mixture. The opening should be a little larger than 1/16inch in diameter. The bigger the opening the bigger the egg threads, and vice versa.
Then hold funnel just above the simmering liquid, remove your finger from the opening and move funnel over stock, letting yolks stream into the hot but not boiling stock. Cook in small batches, lifting out the cooked threads before adding another batch. Put finger over funnel to close; remove to open.