Add the brown sugar, paprika, garlic powder, onion powder, salt, pepper, chipotle powder, and parsley to a small bowl and stir to combine. Remove 1 tablespoon of dry rub and set aside for later.

Coat all sides of the chuck roast with the mustard.

Liberally coat the roast in the dry rub. You may not need all of the rub. Store any excess in a covered container for another use.

Place roast in a slow cooker on low for 8 hours.

Shred the meat with two forks and transfer to a large skillet. Sprinkle with reserved tablespoon of dry rub and add barbecue sauce.

Cook over medium heat for 5 minutes, stirring often.

Serve on sandwich buns with pickles, if desired.