Mix together the sugar, soy sauce, vinegar, hoisin, garlic, ginger and a generous amount of pepper in a medium bowl. Cut the pork belly in half and place in an 8-inch square baking dish with the fatty side up. Pour the marinade over top of the pork belly, cover tightly with foil and refrigerate for at least 2 hours and up to overnight.
Preheat the oven to 300 degrees F.
Bake, covered, until the pork is tender and easy to shred with a fork, about 2 hours. Remove to a cutting board and set aside. Pour the cooking liquid into a medium skillet, bring to a boil over medium-high heat and cook, whisking occasionally, until reduced by half, about 15 minutes.
Meanwhile, steam the bao buns according to the package directions.
Heat the vegetable oil in a small skillet over high heat until very hot. Add the scallions and jalapeño and cook until just starting to turn tender, 1 to 2 minutes. Transfer to a small plate.
Slice the pork belly into 15 pieces. Once the cooking liquid has been reduced, turn the heat off, place the pork belly into the skillet and flip once just to coat with the sauce.
Place a piece of pork belly in each bun, sprinkle with the scallion-jalapeño mixture and garnish with a few cilantro leaves. Serve immediately.
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