Combine the soy sauce, brown sugar, garlic powder, green onions and 4 cups water in a pot. Bring to a boil over high heat, stirring occasionally. Let cool to room temperature. Combine the marinade and the pork belly in a pan, cover and marinate for 24 hours in the refrigerator.
Preheat the oven to 300 degrees F. Set the pan with the pork belly and marinade in the oven. Cook until tender, about 3 hours.
Remove the pan from the oven. Set a second pan directly on the pork belly and weight it with canned goods to compress the pork belly. Refrigerate for at least 6 hours, or until the pork belly is firm.
Crush the roasted peanuts in a food processor or with a mortar and pestle. Mix in the raw sugar. Set aside.
Heat 1 tablespoon oil in a saute pan over medium-high heat. Add the garlic, white onions and preserved mustard greens. Saute for 2 minutes. Stir in the sweet soy sauce. Set aside.
Slice the pork belly into 3-inch-long blocks, and then cut into 1/2-inch-thick rectangles. Sear on both sides in a little oil until golden brown.
Set up a steamer and steam the gua bao until soft and fluffy.
Open up the steamed buns. Spread each with 1/2 tablespoon of the hoisin and then top with a slice of pork belly and 1 tablespoon of the preserved mustard green mixture. Sprinkle with 1 teaspoon of the crushed peanuts and some chopped cilantro.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.