Heat the butter in a heavy-bottomed soup pot over medium-high heat. When it begins to bubble add the sliced and washed leeks. Saute the leeks for abut 2 minutes, or until they are tender and translucent. Add the chicken stock, salt, and pepper, and bring the stock to a boil, then lower it to a simmer. Cook the soup for about 20 minutes, or until the potatoes are easily mashed. Transfer the soup to a blender or food processor, and process it until it is smooth. Return the soup to the pot and bring it again to a simmer. Stir in the cream. Serve hot or cold.