How to Make It
In a bowl, combine the mushroom soy sauce, vinegar, oil and scallion. Grind pepper over the sauce.
Bring a medium saucepan of water to a boil. Remove 20 large outer leaves from the head of cabbage and blanch in the boiling water for 1 minute. Drain and dry on paper towels. Trim the thick stems and discard them. Cut the large leaves into three square pieces (roughly 4 inches) and cut the other leaves into roughly 4-inch squares so you have 32 cabbage squares for filling.
Finely chop enough of the remaining unblanched cabbage to yield 2 cups. Transfer the cabbage to a small colander and toss with 1/2 teaspoon of salt. Let stand until wilted, about 10 minutes. Firmly squeeze the cabbage to remove any moisture.
In a medium skillet, heat the oil. Add the shiitake, season with salt and pepper and cook over moderate heat, stirring, until lightly browned, about 4 minutes.
In a large bowl, stir the oyster sauce, scallions, garlic and ginger. Add the garlic chives, tofu, whole egg, egg yolk, salted cabbage, sautéed shiitake and 1/4 teaspoon of pepper; combine thoroughly.
Lay out the cabbage leaf squares on a work surface. Spoon 1 slightly rounded tablespoon of filling onto each square. Fold in the ends to enclose the filling, making neat squares.
Fill a wok or a very large skillet with 1 inch of water and bring to a boil. Arrange the dumplings in a double-tiered bamboo steamer and set the steamer over the boiling water. Cover and steam the dumplings until they are firm, about 7 minutes. Serve the dumplings right away, passing the dipping sauce at the table.
Mushroom soy sauce is dark soy flavored with mushrooms. It's thick, meaty and salty. If it's unavailable, substitute regular soy sauce.