Prep the slow cooker: Lightly grease the insert or use a liner for easier cleanup.
Add the chopped onion and minced garlic to the bottom.
Make the sauce: In a bowl, whisk together BBQ sauce, chicken broth, apple cider vinegar, brown sugar, Worcestershire sauce, smoked paprika, chili powder, pepper, salt, and cayenne (if using).
Add the chicken: Place the chicken breasts (or thighs) over the onions and pour the sauce mixture on top, ensuring the chicken is coated.
Cook low and slow: Cover and cook on Low for 4–5 hours or on High for 2.5–3 hours, until the chicken is very tender and shreds easily.
Shred the chicken: Transfer the chicken to a large bowl and shred with two forks. Skim excess fat from the sauce if needed.
Finish the sauce: Stir in the butter (if using) to the sauce in the slow cooker for a richer, silkier finish. Return the shredded chicken to the slow cooker and toss to coat.
Cook on Low for another 15–20 minutes to absorb flavor.
Adjust to taste: Add more BBQ sauce for sweetness and body, or a splash of vinegar for brightness. Season with extra salt and pepper if needed.
Serve: Pile onto buns with coleslaw and pickles, spoon over rice, tuck into tortillas, or serve alongside roasted vegetables.