In a dutch oven, cook the leeks and onions in butter over medium heat until lightly browned (about 5 minutes). Add the potatoes, broth, salt and pepper. Boil for 35 minutes, or until the potatoes are very tender. Purée in a blender.
Return mixture to dutch oven. Add milk and ½ cup whipped cream. Bring to a boil. Cool mixture. Purée again. Chill for 3 to 4 hours.
When ready to serve, add remaining ½ cup whipping cream. Mix thoroughly. Serve cold, with a garnish of chopped chives. Yield: 4 servings.
Note : Cold soups require more seasonings than hot; taste the soup after chilling and add extra salt and/or pepper as needed.