In a small bowl, cover the dried mushrooms with 1 cup hot water and let soak for at least 1 hour. Remove the mushrooms from the liquid, chop, and set aside. Strain the liquid through a sieve lined with several layers of cheesecloth and reserve.

In a large saucepan, melt the butter and in it sauté the onion and celery until they soften a bit. Add the lettuce and continue cooking until it wilts, about 3 minutes. Add the stock, the mushrooms, and the reserved mushroom liquid and bring to a boil. Lower the heat and simmer, partially covered, for 30 minutes.

After the mixture cools a bit, purée it in a food processor or food mill until smooth and return it to the saucepan.

In a small bowl, beat the egg yolks into the buttermilk and whisk in several ladlefuls of the hot soup.

Slowly whisk this mixture into the soup and cook slowly and carefully until the soup thickens. Do not allow the soup to boil or the eggs will curdle.

Serve in heated soup bowls.