Preheat your charcoal grill or smoker for indirect grilling, aiming for 225-250F.
Slice the pork belly into manageable strips.
Mix the rub ingredients together. Season all sides of the pork belly generously, allowing it to rest at room temperature for 20-30 minutes.
Place the pork belly pieces in the smoker. I recommend using a wire rack for easy transfer if possible. Smoke for 3 hours.
At least 30 minutes before the pork belly is done with the first phase, make the teriyaki sauce. Mix the water with the cornstarch to create a slurry. Mix all ingredients together in a small pot, capable of being heated over the coals.
Warm the sauce over the heat, whisking occasionally for about 6-10 minutes depending on the heat source. Once it thickens, remove from the heat immediately and allow it to cool.
Remove the pork belly from the smoker at the 3-hour mark. It should be around 160-180F internal temperature. Place it in a small foil pan, and add the sauce with the butter. Stir to combine, and wrap the pan tight with heavy-duty foil.
Place the pan back on the smoker for another hour, up to 90 minutes. The pork should be around 200-205F and probe tender.
Serve on skewers, garnishing with sesame seeds, cilantro, and scallions. (Optional)