Add mayonnaise, buttermilk, parsley, black pepper, salt, garlic powder, onion powder, thyme, and dill to a medium bowl.
Whisk for at least 30 seconds until smooth and no clumps remain.
If too thick, add buttermilk 1 tablespoon at a time until pourable.
Transfer to an airtight container and refrigerate for 30 minutes before serving for best flavor.
Stir before using as dressing separates naturally.
Store refrigerated for up to 1 week.