Add all chicken ingredients (except chicken thighs) to a slow cooker. Whisk until smooth. Trim only the large pieces of fat or gristle from the chicken thighs (no need to remove everything). Place thighs in the slow cooker, flip to coat in the sauce, and arrange in an even layer with the smooth side down. Cook on low for 4–6 hours or high for 2–3 hours.
Once done, remove the chicken, shred with two forks, and stir it back into the sauce.
Slaw: In a large bowl, whisk together mayo, mustard, vinegar, and honey until smooth. Season with about 1/2 teaspoon salt and 1/4 teaspoon pepper. Add cabbage and carrots, then use tongs to toss until well coat. Chill until serving. Toss again before adding to sandwiches.
Spread softened butter (just add to taste) on the insides of the buns. Toast on a skillet or griddle over medium heat until golden.
Add sliced avocado (if using) to the bottom bun. Use a slotted spoon to add the saucy chicken, then top with slaw. Add the top bun and enjoy immediately!
Video
Notes
Storage: Store BBQ chicken and coleslaw separately in airtight containers in the fridge. Both are best within 1–3 days. Toast fresh buns just before serving.