Bring 4 quarts of water to a boil. Add 1 Tbsp salt and the macaroni; cook until VERY soft, about 15 minutes. Drain and return to pot; add the cider vinegar and toss until absorbed. Let cool for 10 minutes.
Meanwhile, whisk together 1 1/2 cups of the milk, 1 cup of the mayonnaise, the brown sugar, 1/2 tsp of salt and 2 tsp pepper.
Once the cooked pasta has cooled for 10 minutes, whisk in the dressing. Let cool completely.
Add the remaining 1/2 cup milk and 1 cup of mayonnaise, along with the scallions, carrot and celery. Stir to combine, then season to taste with salt & pepper. Chill for at least one hour before serving.