Rub yellow mustard all over the Boston butt. Sprinkle salt, ground black pepper and garlic powder over mustard. Place in a large slow-cooker.
Add remaining ingredients, except BBQ sauce, to the slow-cooker on top of the Boston butt.
Cover and cook on medium for 10 to 12 hours or high 8 to 9 hours or until the pork is tender enough to shred with forks.
Remove from slow-cooker to a large sheet pan. Cover with foil and let rest for 30 minutes.
Shred with forks. Add cooking liquid (strained) if necessary to moisten. Add in BBQ to the shredded pork and toss to mix or serve BBQ sauce on the side.