Score the fat cap: Using a sharp knife, cut a crosshatch pattern into the fat cap — cuts 1 inch apart and about 1/4 inch deep. Be careful not to cut too deeply into the muscle.
Apply the dry rub: Mix all dry rub ingredients together. Apply generously to all sides of the pork belly, working the rub into the scored fat cap. For best results, refrigerate uncovered on a wire rack overnight.
Preheat smoker: Set your smoker to 225°F. Add apple wood, cherry wood, or a blend of both. Wait until the temperature is stable and producing clean, thin smoke.
Place pork belly fat side up: Set the slab on the smoker grates fat side up. Insert a probe thermometer into the thickest part of the meat, avoiding the fat layer.
Smoke to 165°F: Smoke at 225°F, spritzing with apple juice every 60 to 90 minutes. Maintain consistent temperature throughout. This phase takes approximately 4 to 6 hours.
Wrap in foil: When the internal temperature reaches 165°F, remove the pork belly from the smoker. Place on a double layer of heavy-duty foil, pour in 1/4 cup apple juice, and wrap tightly.
Continue to 200°F: Return the wrapped pork belly to the smoker at 225°F. Cook until the internal temperature reaches 195-203°F and a probe slides in with almost no resistance. This takes approximately 1 to 2 additional hours.
Rest: Remove from smoker and crack the foil open slightly. Rest for at least 30 minutes, up to 1 hour, before slicing.
Slice and serve: Slice the pork belly about 1/4 inch thick across the grain. Serve immediately.