Assemble the shiu mai:
Pulse the shrimp and scallops in a food processor until almost smooth, about 12
pulses. Transfer to a large bowl, and stir in the sliced cabbage, scallions, cilantro, soy sauce, garlic, rice vinegar, 1 Tbs. cornstarch, ginger, sesame oil, sugar, pepper, and egg white.
Sprinkle a rimmed baking sheet liberally with cornstarch. Set a small bowl of water on the work surface. If the wrappers are larger than 3 inches across in any direction, trim them with a cookie cutter to 3-inch rounds. Otherwise, leave as squares or rectangles.
Working with one wrapper at a time, and keeping the remaining wrappers
covered with plastic wrap so they don’t dry out, place a heaping
teaspoon of the pork filling in the center of the wrapper. Using a
pastry brush or your fingers, dab a bit of water around the edge of the
wrapper to moisten. Crimp the wrapper up and around the filling,
squeezing slightly with your fingers to bring the wrapper together like a
Place on the cornstarch-coated baking sheet, cover with plastic wrap, and repeat with the remaining wrappers and filling until you run out of one or the other. You can steam the shiu mai immediately or freeze and steam them later (see Make-Ahead Tips, below).
Cook the shiu mai:
Set up a steamer with 2 inches of water in the bottom. Line the basket with cabbage leaves to keep the shiu mai from sticking. Set over medium-high heat and cover. When steam begins to escape from the steamer, remove from the heat and carefully take off the lid. Arrange the shiu mai in the steamer so that they don’t touch, as they will stick together (you’ll have to cook them in batches). Cover the steamer and return to medium-high heat. Steam until the pork is cooked through (cut into one to check), 5 to 7 minutes. Serve with the dipping sauce.
Make Ahead Tips
Freeze the uncooked shiu mai on the baking sheet. When frozen, transfer them to an airtight container, setting parchment or plastic wrap between layers, or seal them in a plastic bag. Store in the freezer, where they’ll keep for about a month. Do not thaw the shiu mai before steaming; cooking time will be 10 to 12 minutes.