Place the 2½ pounds boneless, skinless chicken breasts (or thighs) in the bottom of your slow cooker.
In a small bowl, combine 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 Stir until fully blended.
Pour the sauce over the chicken, making sure each piece is evenly coated.
Cover the slow cooker and cook on low for 6–8 hours or high for 3–4 hours, until the chicken is tender and easily shredded with a fork.
Remove the chicken from the crock pot and shred it using two forks. Return the shredded chicken to the sauce and stir until fully coated.
Serve immediately on buns, in tacos, over rice, or however you like. Optionally, top with extra barbecue sauce or pickles for added flavor.