I use salt flour for popcorn; it's essentially salt that is milled as fine as flour. There was a study (which I can't find now) which found that our perception of saltiness is based less on amount and more on surface area coverage.
It's sometimes just called "popcorn salt" and it's exactly as you describe. (And it can also be made at home from regular table salt with a food processor or mortar & pestle.)
I take a trip to the bulk-barn often enough to get “Butter Salt” it’s powdered and yellow. I suspect it’s what goes into the packages for the movie theatre.
