In a large saucepan over medium-low heat, soften the leek in the butter until it turns translucent, about 5 minutes. Add the broth and potatoes. Bring to a boil. Cover, reduce the heat and simmer until the potatoes are tender, about 15 minutes.
Purée the soup in a blender until smooth. Add the cream (see garnish idea). Season with salt and pepper.
Serve hot or cold in glasses or cups.
Instead of adding the cream to the soup, whip it, top each serving with a dollop, sprinkle with pepper and serve immediately.