Heat the butter in a large pot over medium heat. Add the leeks and saute until they wilt, about 6 minutes. Season with slat and pepper. Add the potatoes, broth and water and bring to a boil. Reduce the heat and let simmer until the potatoes are fork tender. When the potatoes are soft, transfer small amounts at a time to a blender or food processor and puree. Stir in the parsley and serve hot.