We air pop popcorn at home. We get kosher salt and grind it down to a fine dust that we keep in a little tub a d sprinkle on the popcorn when we make it.

Very simple and tasty. Much better than drowning it in butter IMO. A bit of garlic powder is also nice.

If you must have butter, what about just melting a little bit and drizzling it on? Using real butter is likely still better than whatever it is they put in those microwave bags.