Melt the butter in a large stockpot over low heat. Add the leek with a pinch of salt and cook slowly for about 10 minutes, or until the leeks are soft.
Add the potato and cook for 3 minutes, then pour in the stock. Bring to the boil and simmer for 20 minutes. Add the cream and cook for another 10 minutes. Using a blender or food processor, purée the soup until smo