Soak the mushroom(s) in the Hot water until softened (about 20 minutes). Drain. Remove and discard the stem and the tough center. Chop very fine. Mix the mushroom(s), pork, shrimp, water chestnuts, green onion(s), soy sauce, sherry, sesame oil, cornstarch, sugar, salt and pepper together. Place a generous teaspoon of the mixture on the center of each won ton wrapper. Bring the sides of the wrappers up around the filling. Pinch the tops together to form bundles around the filling. Press down lightly on the filling. Place the cooking rack and the measured amount of water in the pressure cooker. Place the dumplings on the rack (this may have to be done in batches). Close the cover securely. Place the regulator on top of the vent. Cook for 5 minutes at 15 Pounds - start timing when the pressure regulator begins to rock. Immerse the pressure cooker in or under running cold water to stop the cooking immediately. Remove the dumplings - keep them warm while additional batches are cooked. Serve warm with a dipping sauce such as Sesame Soy Dipping Sauce.