Combine molasses, pickling salt, and water in a 6-quart polycarbonate food storage container. Add Boston butt, making sure it is completely submerged in brine, cover, and let sit in refrigerator for a minimum of 8 hours. 12 hours is ideal.
Place cumin seed, fennel seed, and coriander in a spice/coffee grinder and grind fine. Transfer to a small mixing bowl and stir in chili powder, onion powder, and paprika.
After 8 to 12 hours, remove Boston butt from brine and pat dry. Sift the rub evenly over the shoulder and then pat onto the meat making sure as much of the rub as possible adheres. More rub will adhere to the meat if you are wearing latex gloves during the application.
Heat smoker to 210ºF. Place butt in smoker and cook for 10 to 12 hours, maintaining a temperature of 210ºF. Begin checking meat for doneness after 10 hours of cooking time; you'll know the meat is done when it falls apart easily when pulling with a fork. Once done, remove it from the smoker and set aside to rest for at least 1 hour. Pull meat apart with 2 forks and serve as a sandwich with coleslaw and dressing, as desired.