Make the filling: In a large bowl, combine chicken, water chestnuts, bamboo shoots, scallions, cornstarch, white peppercorn oil, oyster sauce, ginger, Shao Hsing, sugar, soy sauce, sesame oil, salt, pepper, and egg white. Stir to mix thoroughly. Transfer to a shallow dish, and refrigerate, uncovered, for 4 hours, or covered, overnight.
Make the dumplings: Lightly oil a bamboo steamer basket, or line the bottom with a round metal steamer insert; set aside. Work with one wrapper at a time, and cover unused wrappers with a damp cloth. Place 1 1/2 tablespoons filling in center of each wrapper. Hold filling in place with the blade of a butter knife. Holding the dumpling in the other hand, gradually turn the knife and dumpling slowly in a clockwise direction so that the dumpling forms a basket shape.
Remove knife, and pat filling with point of knife to smooth. Squeeze the dumpling slightly to adhere dough to filling, this will ensure that the dumpling and filling will remain intact during the steaming process. Tap the dumpling bottom lightly on work surface to flatten. Place into prepared steamer basket. Repeat with remaining filling and wrappers.
Bring 3 inches of water to a boil in a wok. Place steamer basket over water. Cover, and steam until dumplings are cooked through, about 6 minutes. Turn off heat. Remove the steamer from the wok, and transfer to a platter. Serve dumplings directly from steamer with mustard dipping sauce on the side.