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1. To prepare the roast pork, cut the meat into slices 1½ inches thick. Then cut across the slices every 1 to ½ inches to make long strips. Place the meat in a rectangular glass pan or stainless steel pan that will hold it in one layer. Set the meat aside while preparing the marinade.

2. To prepare the marinade, in a small bowl combine all remaining ingredients and stir well to mix. Pour over the meat and stir the meat around in the marinade so that all the meat is well moistened with it. Cover the pan with plastic wrap and let the meat marinate for 3 to 6 hours at a cool room temperature or in the refrigerator.

3. To roast the pork, set a rack in the middle level of the oven and preheat to 375 degrees.

4. Pour about 1 inch of warm water into the roasting pan and balance the rack on top of the roasting pan. Lift the pieces of pork from the marinade and arrange them on the rack. Roast the pork for about 1 hour, basting 2 to 3 times with the remaining marinade and turning the pieces several times. Remove the pan from the oven and place the rack over a jelly-roll pan. Cool the pork to room temperature. If you are roasting the pork in advance, refrigerate it in plastic wrap before continuing. Cold pork is easier to dice.

5. To prepare the filling, heat the oil in a large saute pan or wok and add the onion. Cook over low to medium heat for about 5 minutes, or until the onion is softened. Stir in the diced pork and chicken stock and simmer 1 minute. Add all the remaining ingredients except the scallions and dissolved cornstarch and stir well to mix. Push the filling aside to expose some of the pan bottom and add the cornstarch mixture. Stir constantly until it thickens slightly, then stir the thickened cornstarch evenly into the filling. Cook, stirring, 1 minute longer, then stir in the scallions. Scrape the filling into a glass or stainless steel bowl and cool it to room temperature. For advance preparation, cover the bowl with plastic wrap and refrigerate it for up to a day before continuing.

6. To prepare the dough, in a small saucepan over low heat. heat the milk until it is lukewarm (about 110 degrees). Remove the pan from the heat and pour the milk into a medium bowl. Whisk in the yeast, followed by the butter. Whisk in the egg with the sugar and salt.

7. Place the flour in the bowl of a food processor fitted with the steel blade. Add the yeast and egg mixture, scraping it in with a rubber spatula. pulse until the dough forms a very soft ball. Let the processor run continuously for 10 seconds to knead the dough.

8. Invert the dough into a buttered bowl and carefully remove the blade. Turn the dough over so that the top is buttered, and cover the bowl with plastic wrap. Let the dough rise at room temperature until it has doubled in bulk, about 1 hour.

9. To form the buns, scrape the dough onto a floured work surface and use a bench scraper or knife to cut it in half. Roll each piece of dough under the palms of your hands to an 8-inch cylinder. Cut each cylinder of dough into eight 1-inch pieces.

10. Place one of the pieces of dough on a lightly floured work surface and press and roll it to a 3-inch disk. Place a heaping tablespoon of the filling in the center and draw the sides of the disk of dough up all around the filling to meet in the center, enclosing the filling. Firmly pinch the seam closed and invert the bun, seam side down, onto one of the prepared pans. Repeat with the remaining dough and filling.

11. Cover the buns with oiled plastic wrap or a towel and let them rise at room temperature until they almost double in bulk, about 30 minutes.

12. About 15 minutes before the buns are completely risen, set racks in the upper and lower thirds of the oven and preheat to 350 degrees.

13. Immediately before baking the buns, carefully brush them with the egg wash.

14. Bake the buns until they are a deep golden color, about 20 to 25 minutes. Switch the pan from the upper rack to the lower one, and the pan from the lower rack to the upper one, about midway through the baking.

15. Cool the buns on a rack.