For the pork belly: Rub the pork belly thoroughly with the rub and refrigerate for 2 to 3 days.
Wash the rub off the pork belly and cut each into 6 squares. Put the garlic, carrots, onions and celery in a large pan. Place the belly pieces on top of the vegetables and add the dark beer. Cover with hot water and add the black pepper, sage and bay leaves. Put the pan over high heat and bring it to a boil. Lower the heat to a simmer and simmer until the pork is super tender and easily breaks apart. Taste for salt and add some if needed before turning the heat off.
For the BBQ sauce: Whisk together the ketchup, brown sugar, honey, molasses, Worcestershire, mustard, Sriracha, liquid smoke, sambal and coriander in a large saucepan over medium-high heat and bring to a simmer. Lower the heat to maintain a low simmer and cook for 45 minutes. Taste and adjust the seasoning if needed.
To assemble the sloppy joes: Warm some of the pork belly meat with some of the BBQ sauce. Serve in hamburger buns.