For the pork belly: Bring a Dutch oven or heavy-bottomed pot of water to a boil. Add the pork belly and simmer for 30 minutes. Remove from the pot, rinse under cold water and then dry with some paper towels. Slice the pork into 3 even pieces and set aside.
Heat a wok over high heat and add the peanut oil. When the oil begins to smoke, add the pork pieces and stir-fry for about 1 minute per side to brown. When the pork is browned, add the dark soy and stir-fry for another 2 minutes. Transfer the pork to a plate that will fit inside a large bamboo steamer with at least a 1-inch margin between the plate and the steamer.
For the sauce: In a small bowl or measuring cup, stir to combine the vegetable stock, light soy sauce, brown sugar and rice wine; set aside. Reheat the wok and add the peanut oil. When the oil is hot, add the ginger, Sichuan peppercorns and star anise and then stir-fry for a few seconds. Add in the sauce and bring to a boil. Remove from the heat and pour the sauce over the pork.
Place the plate of pork into a steamer over a wok or pot filled halfway with water and bring to a boil. Steam the pork over medium heat until the pork is tender, 1 hour to 1 1/2 hours. Check the pot occasionally to be sure that the water hasn't completely evaporated.
Remove the pork from the 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 1 to 2 minutes to thicken the 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, Quick Cucumber and Shallot Pickle, scallions and fresh cilantro.
Put the cucumbers and shallots in a medium bowl.
In a small saucepan, add the mirin, rice vinegar and sugar. Bring to a simmer over medium heat and stir until the sugar dissolves. Remove from the heat and let cool. Pour the vinegar mixture over the vegetables and let marinate for 15 minutes or longer before serving.