Thinly slice the scallions into 1/8-inch rounds, grate the carrot, and finely chop the celery stalks. Set these vegetables aside in separate bowls. Bring a large pot of salted water to a boil and cook the macaroni according to package directions until al dente (usually around 8-10 minutes—I recommend slightly undercooking by about a minute since the pasta will soften further as it cools and absorbs the dressing). Drain the pasta thoroughly but do not rinse it.

While the pasta is still warm, transfer it to a large mixing bowl and immediately stir in the cider vinegar. The warm pasta will absorb the vinegar and develop deeper flavor. Let the pasta cool for about 10 minutes at room temperature, stirring occasionally to ensure even distribution of the vinegar.

In a separate bowl, whisk together the milk, mayonnaise, brown sugar, salt, celery seed, and black pepper until smooth and well combined. I like to add the brown sugar first and let it dissolve in the milk before adding the mayo—this helps ensure the sweetness is evenly distributed throughout the dressing without lumps. This two-stage dressing approach allows the pasta to absorb the tangy vinegar first, then get coated with the creamy dressing for balanced flavor.

Pour the creamy dressing mixture from Step 3 over the vinegar-coated pasta and stir thoroughly until every piece is evenly coated. Add the prepared scallions, grated carrot, and chopped celery, folding them in gently to distribute evenly throughout the salad. Taste the mixture and adjust seasoning with additional salt and pepper as needed—remember that the flavors will meld and intensify as the salad chills.

Transfer the macaroni salad to a serving bowl or storage container and refrigerate for at least 1 hour before serving. This resting time allows the flavors to meld and the pasta to fully absorb the dressing. You can make this salad up to 24 hours ahead—just give it a gentle stir and add a splash of milk if it seems too thick when you're ready to serve, as the pasta continues to absorb moisture.