Melt the butter in a medium-size sauté pan over medium heat. Add the mushrooms and cook, stirring, for 2 minutes. Add the shallots, garlic, 1/4 teaspoon of the salt, and the black pepper and cook another 2 minutes, stirring occasionally. Add the cream and truffle oil if using. Simmer the mixture for 30 seconds. Remove from the heat and let cool completely.
Put the water and 1/2 teaspoon of the salt in a large saucepan and bring to a boil.
Meanwhile, spoon 1 1/2 teaspoons of the mushroom mixture in the center of each won ton wrapper. Lightly brush the edges with water. Fold one corner to the opposite corner to form a triangle. Crimp the edges and seal.
Poach the won tons, several at a time, in the boiling water until soft and tender, about 3 minutes. Remove them from the water with a slotted spoon and drain on paper towels. Season with the pinch of salt.
Yield: 20 dumplings