In a small bowl, mix together the brown sugar, smoked paprika, garlic powder, onion powder, black pepper, kosher salt, and cumin to create the homemade seasoning rub.
Rub the yellow mustard all over the pork belly, then generously apply the homemade seasoning rub, pressing it into the meat. Cover the pork belly and refrigerate overnight.
Preheat the smoker to 225°F. Remove the pork belly from the refrigerator and let it come to room temperature.
Place the pork belly in the smoker and smoke for 6-8 hours, or until the internal temperature reaches 195-200°F. The meat should be tender and easily pull apart.
While the pork belly is smoking, prepare the homemade BBQ sauce. In a medium saucepan, whisk together the ketchup, apple cider vinegar, brown sugar, Worcestershire sauce, smoked paprika, garlic powder, onion powder, salt, and black pepper. Simmer over low heat for 15-20 minutes, stirring occasionally, until the flavors have blended together. Remove from heat and set aside.
Once the pork belly reaches the desired internal temperature, remove it from the smoker and let it rest for 15-20 minutes.
Slice the smoked pork belly into 1/2-inch thick pieces and serve with the homemade BBQ sauce on the side.