Step 1: Remove the skin (also called rind) from the pork belly (if still on). Score both sides with a sharp knife, cutting a 1-inch crosshatch pattern with cuts 1/4 inch deep.
Step 2: Make the rub: Place the salt, sugar, paprika, garlic, chili powder, cumin, black pepper, mustard powder, white pepper, and cayenne in a bowl. Mix well with your fingers.
Step 3: Sprinkle the rub over the top, bottom, and sides of the pork belly, rubbing it into the meat with your fingers.
Step 4: Set up your smoker following the manufacturer’s instructions, and preheat to 225 degrees. Add the wood as specified by the manufacturer.
Step 5: Smoke the pork belly fat side up until bronzed with smoke and the internal temperature is 165 degrees. This will take about 3 to 4 hours.
Step 6: You can serve the pork belly hot out of the smoker, but you’ll get a better texture and flavor if you chill and reheat it. If using the second option, transfer the pork belly to a wire rack over a baking sheet and let cool to room temperature, then cover and refrigerate until cold. The recipe can be made up to 3 days ahead.
Step 7: Cut the chilled pork belly crosswise (against the grain) into 1/2-inch thick slices. Lightly season each slice on both sides with salt and pepper.
Step 8: Meanwhile, set up your grill for direct grilling and preheat to medium-high. Ideally, you’ll be grilling over wood or will add some wood chunks to your fire.
Step 9: Arrange the pork belly strips on the grate on the diagonal and grill until sizzling and browned on both sides, about 2 minutes per side. (Give each slice a quarter turn after 1 minute to lay on a crosshatch of grill marks.) Don’t over-crowd your grill. The melting fat may cause flare-ups—so you want to leave yourself plenty of maneuvering room to dodge the flames.