Melt butter in a skillet and add leek. Sauté leek in butter for 4 minutes. Separate.
Place water or bouillon in a large saucepan and add chopped potatoes, salt and leeks. Bring to a boil and cook until everything is tender, approximately 50 minutes to 1 hour.
Transfer all to a processor or a blender and process to form a purée. Correct seasoning.
Stir in heavy cream just before serving.
Garnish with chopped parsley.