Heat your smoker to 225°F and set up for indirect heat; we use lump charcoal and a few chunks of apple or hickory wood (you can also use chips but you’ll need to replace them a few times during smoking).
Trim off the fat cap—there is so much fat and marbling running through the meat, you don’t need the cap and the pork will get a better bark without it. Wiggle the tip of a knife underneath the fat so loosen it and use your other hand to pull the fat up and away as you slice between the fat and the meat (don’t worry about getting every piece; just take off a good amount of it). Pat the pork dry with paper towels. Slather all over with mustard.
In a bowl, whisk together the salt, brown sugar, paprika, chili powder, cumin, garlic powder, onion powder, black pepper, and cayenne pepper. Rub liberally all over the pork. Transfer the pork to a disposable aluminum roasting pan (this is optional but the pork really makes a mess on the smoker, so we like to use a pan for easy clean up).
Smoke the pork for 10 to 15 hours– it’s a big range of cooking times, because in addition to the weight of your pork, everything from your smoker model, to how you built your fire, to the temperature outside that day will affect the time. A general guideline is 1.5 hours per pound. Pork needs to rest and can stay hot for hours, so give yourself a big buffer. At the 3-hour mark, mist it all over with apple juice, then mist it again every few hours. To speed up the smoke time, you can wrap the pork in a double layer of heavy-duty foil once it reaches an internal temperature of 175°F. If the pork gets "stuck" for several hours at the same temperature, don't panic. This is called "the stall" and is expected when smoking pork butt (it usually stalls around 140° to 165° and can even stall again around 180°F; it depends on the pork). If your smoker temperature fluctuates, don't stress, pork butt is forgiving; try to keep it between 225° and 250°F.
Smoke the pork until it reaches 195°F on an instant read thermometer; check in several place. The probe should slide in and out super easily; if it feels tough or tight, smoke for another 30 minutes, then check again. Repeat as needed (it's really hard to overcook pork shoulder).
Cover the pork (if it is not already) and let rest AT LEAST 30 minutes—1 hour is preferable. Carefully shred, discarding any large, tough chunks of fat and the bone. Enjoy!
Notes
TO STORE: Leftover smoked pork butt can be stored in an airtight container in the refrigerator for up to 4 days.
TO REHEAT: Reheat pulled pork in a skillet over medium heat, stirring occasionally until heated through. You can also reheat it in the microwave in a loosely covered container.
TO FREEZE: To freeze pulled pork, transfer it to in an airtight container or freezer-safe bag. Thaw overnight in the refrigerator before reheating according to the instructions above.