Blanch cabbage leaves in a large pot of boiling water until softened, about 1 minute, then drain well in a colander.
Bring all aspic ingredients to a boil, uncovered, in a 4-quart heavy saucepan, stirring until sugar is dissolved, then simmer, uncovered, until liquid is reduced to about 1/2 cup, about 2 hours. Pour liquid through a fine-mesh sieve into a bowl, discarding solids. Pour liquid into a measuring cup and let stand until fat separates from broth, about 1 minute. (If you have less than 1/2 cup broth, add water. If you have more, boil broth to reduce it to 1/2 cup.) Chill broth, covered, until it jells into a solid aspic, about 6 hours.
Lightly scrape off any fat from aspic with a spoon and discard, then coarsely chop aspic. Chop ground pork with a cleaver or a large heavy knife until very finely minced and fluffy.
Beat together aspic, pork, and remaining filling ingredients with an electric mixer at medium speed until combined well, about 30 seconds. Form filling into 24 mounds (2 teaspoons each) on a plastic-wrap-lined baking sheet and cover with another sheet of plastic wrap. Chill while making bun wrappers.
Make wrapper dough:
Put 2 cups flour in a medium bowl, then add boiling water and stir with a fork until a shaggy dough forms. Turn out onto a lightly floured surface and knead until smooth and elastic, about 5 minutes (dough will have the texture of a slightly sticky marshmallow). If dough is too sticky, knead in more flour, 1 tablespoon at a time. Wrap dough in plastic wrap and let stand at room temperature at least 10 minutes and up to 30 minutes.
Make dipping sauce while dough is standing:
Stir together vinegar and ginger.
Assemble and steam buns:
Line steamer rack with cabbage leaves.
Form dough into a snakelike roll about 12 inches long on a lightly floured surface, then cut crosswise into 24 equal pieces with a floured knife and cover with plastic wrap.
Have a small bowl of water ready. Keeping remaining dough covered, place 1 piece of dough cut side down on a lightly floured surface and flatten slightly to form a round. Pick up flattened round and move hands around edges of dough (like turning a steering wheel), allowing dough to stretch slightly, until about 3 inches in diameter. Using small rolling pin with one hand and rotating dough round with other hand, gently even out dough round until it is 3 1/2 to 4 inches in diameter. (Do not roll out center 1 inch of dough; center of round should be slightly thicker than edge.)
Lightly moisten outer 1/4 inch of dough round with a finger dipped in water, then put 1 mound of meat filling in center of round. Pleat edge all around, then pinch and twist pleats together. Place bun on cabbage in steamer and cover with steamer lid to keep bun from drying out. Make more buns with remaining dough and filling in same manner and arrange in steamer, spacing evenly in 1 layer and covering with lid.
Fill wok with enough water so that bottom rim of steamer (not rack) will rest in water. Bring water to a rolling boil over high heat (without steamer in wok), then place steamer in wok and steam buns, covered with steamer lid, over boiling water until buns are firm (not gummy) to the touch and skins are slightly translucent, 10 to 12 minutes. Serve immediately, with dipping sauce.