Heat butter in a large saucepan over medium-high heat. Add onion and cook until softened, about 5 minutes. Add the potatoes and stir in the broth. Bring to a boil, lower the heat and simmer until potatoes are very soft, about 45 minutes.
Meanwhile, finely chop 1/2 of the candy bar. Freeze the remaining half and just before serving shave with a vegetable peeler or large grater to garnish the soup.
Puree the potato/broth mixture in a blender, then add in the chopped candy and continue to blend. Return the pureed mixture to the saucepan. Add in the cream, milk, sherry, nutmeg, and cinnamon. Season with salt and pepper, to taste.
Ladle the soup into bowls and garnish with canned whipped cream and shaved candy. Serve immediately.
Copyright 2002, Janette Barber and Christina Deyo, all rights reserved