Pulse cabbage and carrots in a food processor until very fine (rice-sized, not paste)
In a bowl, whisk mayonnaise, sugar, vinegar, milk, buttermilk, salt, and pepper until smooth and slightly thin
Add cabbage and carrots to a large bowl and pour dressing over
Mix thoroughly until evenly coated
Cover and refrigerate at least 4 hours, preferably overnight
Stir well before serving and adjust seasoning if needed
Notes
Texture is key: vegetables must be very finely choppedDressing should look slightly loose before chillingDo not use pre-shredded coleslaw mixStir after chilling if liquid separates slightlyBest flavor and texture develop after long chilling