Step 1: Remove the skin from the pork belly (if still on). Score both sides, cutting a 1-inch crosshatch pattern with cuts 1/4-inch deep.
Step 2: Sprinkle the rub over the top, bottom, and sides of the pork belly, rubbing it into the meat with your fingers.
Step 3: Set up your smoker, following the manufacturer’s instructions, and preheat to 225 degrees F. Add the wood as specified by the manufacturer.
Step 4: Smoke the pork belly fat side up until bronzed with smoke and the internal temperature is 165 degrees F. This will take about 3 to 3-1/2 hours.
Step 5: Cut the pork belly crosswise (against the grain) into 1/2-inch thick slices. Lightly season each slice on both sides with salt and pepper.
Step 6: Meanwhile, set up your grill for direct grilling and preheat to high. Ideally, you’ll be grilling over wood; or add some wood chunks to your fire.
Step 7: 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—you want to leave yourself plenty of maneuvering room to dodge the flames.
Step 8: To finish the steamed buns, fill each with sliced pork belly (1- by 2-inch squares). Add a spoonful of Hoisin Barbecue Sauce. Top with cucumber, scallion, and sesame seeds.