Make Braised Pork - Cut pork into long 2-inch slabs. Bring enough water to cover pork. Boil for 2 minutes and drain in a colander. Pat dry pork and set on a plate. Wipe out bottom of same skillet and and set over medium-high heat. Heat vegetable oil and fry pork slabs on all sides to brown, about 2 minutes per side. Transfer back to plate.Fry garlic and ginger about 45 seconds. Add star anise, 1 chili pepper, white pepper, and brown sugar. Cook, stirring, about 2 minutes to melt sugar. Add in rice wine and bring to a boil, stirring to dissolve sugar, about 2 minutes more. Add light and dark soy sauces, and scallions. Place pork slices back in skillet and cover with water by ½-inch, and bring to a boil. Reduce heat to low, cover skillet, and cook until pork is very tender, about 1 hour. Make Dough - In a stand mixer with dough hook, mix all dry dough ingredients for 30 seconds. Add wet liquids and knead 9-12 minutes on medium low to get a smooth and soft dough. Shape into a large ball in the mixer bowl, cover with a moistened kitchen towel and leave on counter for an hour to proof and increase in size by 50%. Transfer to a lightly floured work surface and knead again to get smooth surface again. Divide into equal sized portions. Knead each portion for a minute each. Flatten each bun by hand and lightly brush top with oil. Fold in half to make a clam shell.
Steam Buns - Bring a large pot or wok with a steamer insert to a boil. Place buns into the steamer, cover, and heat through for 2-3minutes. Turn heat off for 15 minutes. Bring back to a boil and then steam for another 10 minutes. Keep covered until ready to use.
Prepare Toppings - Combine nuts and white sugar in a food processor. Pulse to create a coarse powder, about 1 minute. Rinse and finely chop mustard greens. Separate cilantro stems.
Assemble and Serve - When pork has cooked, remove and slice slabs into ½-inch slices. Take a steamed bun and fill it with chopped mustard greens, pork slices, and sprinkle with peanut powder. Lay a cilantro sprig on top and serve hot with sriracha chili sauce on the side.