Set out the mayonnaise, sour cream, apple cider vinegar, sugar, Dijon mustard, celery seed, salt, black pepper, and milk on your counter. Measure each ingredient before you start.
Pour the apple cider vinegar into a medium mixing bowl. Add the granulated sugar and fine salt, then whisk for 30 to 45 seconds until mostly dissolved and no longer gritty.
Add the mayonnaise and sour cream. Whisk slowly at first, then faster for about 1 minute until smooth and uniform. Scrape down the sides and bottom of the bowl.
Add the Dijon mustard, celery seed, and black pepper. Crush the celery seed lightly between your fingertips as you add it, then whisk for about 20 seconds until evenly speckled.
Taste the dressing. If too thick, whisk in 1 tablespoon of milk or buttermilk. Adjust tang with a splash of vinegar or sweetness with a pinch of sugar. The dressing should taste slightly bolder than desired since cabbage will mellow it.
Cover the bowl or transfer to a mason jar. Refrigerate for at least 30 minutes, or up to overnight. Stir before using since it may thicken slightly.
Empty the coleslaw mix into a large mixing bowl. If shredding your own, remove outer leaves and core, slice cabbage thinly, and stir in 2 grated carrots. Pat dry if wet and break up any clumps.
Pour about three-quarters of the chilled dressing over the cabbage. Toss with tongs for about 1 minute until every shred is lightly glossy. Add remaining dressing if desired. Serve right away or refrigerate for 30 minutes to 1 hour for a softer texture.