A cold creamy potato and leek soup, vichyssoise is often erroneously thought to be of French origin. The soup was actually developed by Louis Diat, chef at the Ritz-Carlton Hotel in New York, sometime between 1910 and 1920. Potatoes and leeks are cooked in chicken broth or stock, and the mixture is puréed until smooth. Cream is added and then the soup is chilled.