In a large mixing bowl, combine mayonnaise, apple cider vinegar, white sugar, salt, black pepper, and cayenne pepper. Whisk until smooth and well combined.

If using red onion, place diced pieces in a fine-mesh strainer and soak in cold water for 20 minutes to reduce harshness. Drain well and add to the dressing mixture.

Using a box grater, grate carrots using light pressure to create small, fine shreds rather than long strips. This ensures better distribution and more surface area for flavor. Add grated carrots to the dressing and mix well.

Bring a large pot of well-salted water to a rolling boil. Add elbow macaroni and cook according to package directions (typically 5 minutes), stirring frequently to prevent sticking.

Drain cooked macaroni in a colander, then rinse thoroughly with cold water to stop cooking and remove excess starch. Let drain for 20 seconds.

Add the warm, drained macaroni to the dressing mixture. Stir thoroughly until evenly coated. The mixture will look very saucy – this is correct! Cover with plastic wrap and refrigerate for at least 3 hours or overnight.

Remove from refrigerator and stir well. Add 1-2 tablespoons of fresh mayonnaise and 2-3 tablespoons of cold water to restore creaminess. Taste and adjust seasoning if needed.

Transfer to serving bowl and garnish with additional grated carrot, minced red onion, and fresh chives if desired.