Put the chicken in a 5-quart slow cooker and add the hot sauce, broth, garlic powder and salt.
Cover and cook on low until the breasts reach 165°F, 3 1/2 to 4 hours.
Transfer the chicken to a shallow bowl, shred it and whisk the butter into the hot cooking liquid.
Return the chicken and toss until evenly coated, then hold on warm for 10 minutes.
Mix the cabbage, celery, mayonnaise and vinegar and season the slaw lightly with salt.
Toast the rolls cut-side down in a dry skillet if desired.
Fill each roll with buffalo chicken and a small mound of celery slaw.