Cook the pasta in salted water per the directions until al dente. Drain well.
In a large mixing bowl, whisk together the dry buttermilk ranch dressing, 1 cup mayo and 1 ½ cup buttermilk. Set aside. [See Notes]
Mix the drained pasta with the dressing, cubed chicken, crumbled bacon, cherry tomatoes, chopped purple onion, diced eggs, bleu cheese crumbles and green onion. Save some bleu cheese, bacon and green onion for garnishing. Taste and adjust the salt and pepper to your taste.
Chill thoroughly. Add the diced avocado just before serving. Serves: Main dish [6-8] Side dish 
You may substitute 2 ½ cup good quality prepared buttermilk Ranch dressing and omit the dry Buttermilk Dressing Mix mix, mayo and buttermilk, if desired.