Put the mayonnaise, buttermilk, and buttermilk powder in a medium jar with a tight-fitting lid. Sprinkle with a little salt and lots of pepper. Add the chives or parsley if you like, put on the lid, and shake vigorously for 30 seconds or so.
Taste and adjust the seasoning. Serve or cover and refrigerate for up to 3 days. (It will keep longer if you don’t add the fresh herbs.)