Boil the macaroni – Bring a dutch oven to a rolling boil with heavily salted water. Cook the pasta until very soft (about 12 minutes), not al dente. Drain well. Pour the drained pasta into a large bowl. Do not rinse the pasta under any hot or cold water. You want the starchy exterior to remain. 

Add vinegar – Immediately toss the hot macaroni noodles with apple cider vinegar and let it cool for 10 minutes. Stir the vinegar soaked pasta every couple of minutes as it cools to evenly distribute the vinegar into the noodles and quickly cool it. 

Make the dressing – In a separate bowl, whisk together mayonnaise, milk, sugar, salt, ground pepper. Keep it in the refrigerator until ready to dress your mac salad. 

Mix and chill – Once the pasta has cooled, stir in 3/4 the dressing until everything is evenly coated. Add in the shredded carrots, minced celery, celery seeds, and thinly sliced green onion. Season with salt and pepper to taste. Chill for at least 1 hour, or ideally overnight. 

Taste and adjust – Just before serving, add in the remaining 1/4 of the reserved salad dress, and give it a stir. This will keep the salad extra creamy! Store in an airtight container.