Empty bag of coleslaw mix in a large bowl and set aside.
In a medium bowl, mix together mayonnaise, buttermilk, vinegar, dry mustard, celery seeds, salt, black pepper, and cayenne pepper.
Pour mixture over slaw mix and toss to coat. If desired, add more cayenne.
Cover and refrigerate for at least 1 hour or up to 1 day before serving. Refrigerate any leftovers.