Make the filling In a medium bowl, stir or whisk the broth, oyster sauce, ketchup, sugar, cornstarch, soy sauce, 1/2 tsp. salt, and a pinch of pepper. Finely dice enough of the barbecued roast pork to yield 1-1/2 cups (about 6 oz.). Heat a wok over high heat for 30 seconds. Add the peanut oil and swirl to coat. When a wisp of white smoke appears, in about 30 seconds, add the onion. Reduce the heat to medium low and cook, stirring often, until golden-brown, about 6 minutes. Add the pork, increase the heat to high, and stir-fry to combine, 2 to 3 minutes. Drizzle the wine from the edge of the wok into the pork mixture and stir well. Reduce the heat to medium. Make a well in the center of the mixture and pour in the sauce. Stir until the mixture thickens, about 3 minutes. Add the sesame oil and mix well. Refrigerate until cool. Make the dough Mix the flour, sugar, and baking powder on a clean work surface and make a well in the center. While slowly pouring the milk into the well, use your fingers in a circular motion to pull the flour mixture into the milk until it’s absorbed. Make another well, add 3 Tbs. room-temperature water, and continue to use your fingers to work the dough. Add the lard or peanut oil and, using your fingers and a dough scraper or bench knife, work the dough until thoroughly combined. Gather the dough with the dough scraper in one hand and begin kneading with the other. Knead the dough for 10 to 12 minutes—it should feel smooth, pliable, elastic, and slightly tacky to the touch. If the dough is too sticky to work with, sprinkle a little flour on the work surface and your hands as you knead it. If the dough feels dry, lightly wet your hands with water and continue kneading. When the dough is smooth and elastic, shape it into a ball, cover with a slightly damp cloth, and let rest at room temperature for about 1 hour. (The dough must be used within 2 hours of the time it was made. It cannot be frozen.) Portion the dough Have ready sixteen 2-1/2-inch squares of parchment or waxed paper. Lightly flour a work surface. Roll the prepared dough into a 16-inch-long log. Cut the log into 16 equal pieces and then roll each piece into a ball. Cover the dough with the damp cloth. Make the buns Working with one piece at a time, shape a dough ball into a cup that’s about 1-1/2 inches deep and about 3 inches in diameter. The sides of the dough cup should be thinner than the bottom. Hold the dough cup in one hand and spoon about 1 Tbs. of the pork filling into the center. Gather the edges of the dough and pull them up and over the filling, using your thumb to push the filling down as you pleat with your fingers to cover the filling. It may seem like a tight fit at first, but the dough will stretch as you pull it around the filling. Twist the top to seal the bun and pinch off any excess dough. Put the bun, knot side up, on a parchment square and set aside. As you gain confidence, you may use 1-1/2 Tbs. of filling in subsequent buns. Repeat until 16 buns have been made, cleaning off your thumb on a damp cloth after making each bun. Divide the buns (still on their parchment squares) equally between 2 bamboo steamers , spacing the buns at least 2 inches apart. Stack the steamers on top of each other and cover. In a wok, bring 6 cups of water to a boil over high heat. Set the stacked steamers over the boiling water and steam the buns until they look fluffy and their tops have opened like flowers to slightly reveal the filling, 15 to 20 minutes. Remove the steamers from the wok, put them on platters and serve the buns immediately, straight from the steamers.
