Marinate the pork belly. Cut slits into the skin of the pork belly about ¼ inch deep and ½ inch apart. Combine soy sauce - lime juice in a small saucepan and bring to a simmer over medium heat. Cook until the sugar has melted, stirring frequently. Let cool. Place the pork belly skin side down in a container and pour the marinade. Let sit overnight, turning the pork belly over occasionally.

Preheat oven to 325 degrees F.

Transfer pork to an oven-safe dish with 2 cups marinade and 2 cups water. Cover with foil. Bring to a boil over the stove-top. Continue cooking the pork in the oven for 2 ½ - 3 hours or until the pork is tender and falls apart easily. Let cool. When the pork is cool enough to handle, slice into ¼ inch thick slices. Set aside.

While the pork is braising, pickle the cucumbers and shallot. Combine the mirin, rice vinegar, and sugar together, mixing until the sugar is dissolved. Pour over the cucumber and shallot in a medium-size jar. Let chill for at least 30 minutes or overnight.

Prepare the peanuts by crushing together the peanuts with the brown sugar with a mortar and pestle or with a knife. Crush until the peanuts are well combined with the sugar.

Heat the bao buns in a steamer for 10 minutes or until soft and warm. Assemble the buns with slices of the pork belly, pickled cucumbers and shallots, cilantro, and peanut crumble. Serve immediately.