Mix the ingredients for the marinade together in a Ziploc bag. The maltose is a little tough to incorporate but it's okay if there are some lumps as these will eventually dissolve, just make sure there are no big clumps.
If your pork belly has skin, use a sharp knife to remove it. Add the pork belly to the marinade and push out as much air as possible so the meat is completely surrounded by marinade. Let it sit in the fridge for at least 2 days, flipping the bag over every to ensure it's evenly marinated.
To roast your char siu, preheat the oven to 275 degrees F and move the rack to the upper middle position. Set an elevated wire rack on a rimmed baking sheet and lay the marinated pork belly on the rack, saving the marinade for later. Put the pan in the oven and let it roast for 1 hour or until the meat reaches an internal temperature of 160 degrees F. Remove the pan from the oven, then move the oven rack to the top position and turn the heat up to "broil".
Baste the pork with the reserved marinade, then broil it until dark and glossy with the edges just slightly charred. Flip the meat over and baste again, allowing the second side to color and char as well.
Slice your finished char siu and serve with rice or noodles.