Melt the butter in a 6-quart stockpot over medium-high heat. Add the onions, celery, salt, cayenne, and black pepper and cook, stirring, until the vegetables are very soft and lightly golden, about 8 minutes. Add the mushrooms, bay leaves, and garlic and cook, stirring, for 2 minutes. Add the stock and potatoes and bring the mixture to a boil. Reduce the heat to medium and simmer, uncovered, until the potatoes are very soft and the mixture is thick and creamy, about 1 hour and 15 minutes.
Remove the soup from the heat. Discard the bay leaves. With a handheld emersion blender, puree the soup until smooth. Slowly add the cream and truffle oil. Stir gently to blend.
Ladle the soup into individual bowls, add two dumplings to each bowl, and top with the fried parsley and shavings of truffle, if you wish.
Yield: 10 servings