Bring a casserole dish or heavy bottom pot of water to a boil. Add the pork belly and simmer for 30 minutes. Remove from pot and rinse under cold water, and then dry with some paper towels. Slice pork into three even pieces and set aside.
Heat a wok over high heat and add one tablespoon of the peanut oil. When the oil begins to smoke, add pork pieces to wok and fry for about one minute per side to brown. When pork is browned, add dark soy and stir-fry for another two minutes. Transfer pork to a plate that will fit inside a large bamboo steamer with at least a 2.5cm margin between the plate and the steamer.
In a small bowl or measuring cup, stir to combine the vegetable stock, light soy sauce, rice wine and brown sugar; set aside. Reheat the wok and add the second tablespoon of peanut oil. When the oil is hot, add the ginger, Sichuan peppercorns, star anise and then fry for a few seconds. Add in the sauce mixture and bring to a boil. Remove from heat and pour the sauce over the pork.
Place the pork filled steamer over a wok or pot filled half way with water and bring to a boil. Steam the pork in the sauce on medium heat for one hour to 1-1/2 hours until pork is tender. Check pot occasionally to be sure that the water hasn’t completely evaporated.
Remove pork from steamer and transfer all of the juices from the pork into a small wok or pan. Bring to a boil over medium-high heat and stir in the cornstarch slurry. Cook for one to two minutes to thicken sauce.
In a mortar and pestle, crush the peanuts, or finely chop them with your knife. Combine the peanuts and brown sugar; set aside until ready to serve.
Slice the pork belly. Fill each bao bun with a slice of pork and a few tablespoons of the thickened pork sauce, garnish with the sweetened peanut crumble, cucumber and shallot pickle, scallions, and fresh coriander.
For the Quick Cucumber and Shallot Pickle:
Put sliced cucumber and shallot in a medium bowl.
In a small saucepan add remaining ingredients. Bring to a simmer over medium heat and stir until sugar dissolves. Remove from heat and let cool. Pour vinegar mixture over vegetables and let marinate for 15 minutes or longer before serving.