Make the pork belly marinade by combining soy sauce, calamansi or lemon juice, brown sugar, ketchup, garlic, and ground pepper (to taste) in a bowl.
Marinate the pork belly slices in the fridge for 4 to 8 hours.
Preheat the oven to 425° F (218° C). Line a baking sheet with double layers of aluminum foil to make cleanup easier.
Arrange pork belly slices in a single layer and bake them for 20 minutes.
Bring any marinade that touched raw pork to a boil before using it as a basting sauce. Then reduce the heat and cook until slightly thickened, about 10 minutes.
Remove the meat from the oven and brush it with basting sauce on both sides. Return it to the oven for another 20 minutes. Adjust baking time based on the thickness of the meat. The thinner the slices, the faster they cook.
To caramelize the outside, broil on low for about 3 minutes. Be careful not to burn them.
Cut into bite-size pieces and serve them with a dipping sauce by combining vinegar, shallot, chilies, sugar, salt, and pepper to taste.