Place the 2½ pounds boneless, skinless chicken breasts (or thighs) in the bottom of your slow cooker in a single layer.
In a bowl, whisk together 1½ cups barbecue sauce, 2 tablespoons yellow mustard, ¼ cup brown sugar, 2 tablespoons apple cider vinegar, 1 teaspoon garlic powder, 1 teaspoon onion powder, 1 teaspoon smoked paprika, ¾ teaspoon salt, and ½ teaspoon black pepper until smooth.
Pour the sauce over the chicken, turning to coat each piece evenly.
Cover and cook on LOW for 6–8 hours or HIGH for 3–4 hours, until the chicken is fork-tender and reaches 165°F.
Shred the chicken with two forks directly in the sauce and stir to coat. Let it rest 10–15 minutes to soak up the sauce.
Serve on buns, in tacos, over rice, or however you like. Top with extra sauce or pickles if desired.