Whisk the sauce. In a bowl, combine BBQ sauce, broth, apple cider vinegar, brown sugar, Worcestershire, smoked paprika, garlic powder, onion powder, salt, pepper, and red pepper flakes.
Add to the slow cooker. Place the chicken in the slow cooker, then pour the sauce over the top.
Turn to coat the chicken evenly.
Cook low and slow. Cover and cook on Low for 4–5 hours or High for 2–3 hours, until the chicken is cooked through and shreds easily with two forks.
Shred the chicken. Transfer the chicken to a cutting board and shred with two forks, or shred directly in the slow cooker if it’s easy to handle.
Reduce and enrich (optional). If the sauce looks thin, leave the lid off and switch to High for 15–20 minutes to thicken slightly. Stir in the butter for a glossy, velvety finish.
Return and toss. Add the shredded chicken back to the sauce. Toss until everything is coated and juicy.
Taste and adjust salt, sweetness, or heat as needed.
Serve. Pile onto toasted buns, top with coleslaw, or spoon over rice, baked potatoes, or roasted veggies.