Peel and wash one pound of potatoes, cut them in small cubes and put them in a saucepan with two leeks also cut in small pieces. Add a little butter, salt and pepper. Cook this gently for about ten minutes, and add about three quarts of water. Boil very quickly and, when the vegetables are well cooked, pass it all through a fine sieve. Meanwhile, melt a little butter in a saucepan and cook in it a quarter of a pound of sorrel, cleaned and chopped. When the butter has been absorbed, add the soup and keep it boiling. Put into a hot soup tureen the beaten yolks of three eggs mixed with a port wineglassful of cream. Add the boiling soup, having seen that it is well seasoned. Stir well and serve at once.