In a medium bowl, combine the mayonnaise, sour cream, buttermilk, kosher salt, black pepper, garlic powder, onion powder, dried chives, dried dill, and dried parsley.
Whisk with a hand whisk until smooth.
Serve immediately, or cover and refrigerate the dressing for an hour to allow the flavors to meld.
Keep the leftovers in the fridge for up to a week.
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Notes
This dressing is excellent right after you've mixed it, so you can definitely serve it right away. But if you let it rest in the fridge, covered, for an hour or so, allowing the flavors to meld, it gets even better. Just remember to give it another whisk before using it.