In a large mixing bowl, whisk together the mayonnaise, buttermilk, apple cider vinegar, sugar, Dijon mustard, and celery salt until completely smooth and well combined. This is your dressing base.

Taste the dressing and adjust seasoning if necessary; add a pinch of salt or pepper to your preference.

Add the shredded cabbage mixture and carrots (if using) to the dressing. Toss gently but thoroughly until all the vegetables are evenly coated. Ensure you do not overmix, which can wilt the cabbage.

Cover the bowl and chill the coleslaw in the refrigerator for at least 30 minutes (or up to 4 hours) to allow the flavors to meld and the slaw to slightly soften.

When ready to serve, lightly toast the inside of the brioche buns.

Assemble the sandwiches: Place a generous portion of warm BBQ meat onto the bottom bun. Top heavily with a portion of the chilled, creamy coleslaw. Place the top bun on and serve immediately.