Diacetyl is a naturally occurring substance that gives butter its characteristic flavor and aroma, and is often a component of artificial flavoring formulations. Acetoin, a closely related chemical, also occurs naturally and is an ingredient in many flavoring formulations, perfumes and essences.
Artificial Butter Flavorings (ABF) Components
https://www.popcornsupply.com/flavacol-butter-flavored-seasoned-salt
So what makes the buttery flavor?