
In a pot over medium heat, bring about 4 quarts of water to a boil. Add elbow macaroni and cook according to the package's direction. Drain well and transfer to a large bowl.
While still hot, drizzle the apple cider vinegar. Allow to cool.
In another bowl, combine mayonnaise, milk, brown sugar, salt, and pepper. Whisk together until smooth.
Add the carrots and onions to pasta. Gently toss to evenly distribute.
Add 1/2 of the dressing to the macaroni mixture, and stir to combine. Season with salt and pepper to taste. Let stand for a few minutes to allow pasta to soak up the dressing.
Add the remaining half of the dressing and stir to fully coat. Garnish with chopped green onions, if desired.
Cover the bowl with a tight-fitting lid and refrigerate for about 2 to 3 hours to completely chill and the flavors to meld. Serve cold.