How to Make It
In a small bowl, whisk the soy sauce with the black vinegar and chili-garlic sauce.
In a medium bowl, combine the pork and shrimp with the scallions, ginger, soy sauce, sesame oil, Shaoxing wine and salt. Cover with plastic wrap and refrigerate for 4 hours.
Lightly dust a baking sheet with flour. Hold a dumpling wrapper in your palm, keeping the rest of the wrappers covered with plastic. Place 1 tablespoon of the filling in the center of the wrapper and gather up the edges all around to form an open cup; the wrapper will adhere to the filling. Transfer the dumpling to the prepared baking sheet and cover with plastic wrap. Repeat to form the remaining dumplings.
Fill a wok or large skillet with 3/4 inch of water and bring to a simmer. Line 2 tiers of a bamboo steamer with cabbage leaves, overlapping them slightly. Add the dumplings to the steamer and stack the tiers. Cover and set the steamer in the wok. Steam the shumai until firm and the filling is no longer pink, 8 to 10 minutes. Garnish each with a pea and serve hot with the dipping sauce.