Melt the butter in a flameproof casserole or Dutch oven and sautÄ the leeks and potatoes for 10 minutes over a low heat, stirring occasionally. Pour in 1 quart of water. Bring to a boil and simmer for 30 minutes. Strain the vegetables, reserving the cooking liquid. Add enough water to the cooking liquid to make 8 cups of soup. Return the vegetables to the soup and return to a boil, then remove the pan from the heat. In a small bowl whisk together the egg yolks and crÅme fraöche and add the mixture to the soup. Do not boil the soup. Stir the cockles into the soup just before serving.
