Refrigerate the dressing base in an airtight container, up to 2 weeks, before adding mix-ins. For a lower-fat version, replace the sour cream with low-fat plain Greek yogurt.
In a medium bowl, whisk together buttermilk, sour cream, and mayonnaise until smooth. Whisk in avocado, chives, lemon juice, and parsley. Season to taste with salt and pepper.