Usually movie theaters use butter-flavored oil, which has a lower water content than butter so it makes popcorn less soggy—so now you know: water contained in butter is the culprit! But you really don’t need to buy additives to make your own delicious buttery popcorn at home.
To make clarified butter, melt 2 sticks butter in a pyrex measuring cup in the microwave. Allow to sit for a few minutes; the butter will separate into 3 layers. Skim off the top layer of foam with a tablespoon, then slowly pour the clarified butter into a heatproof container; discard the bottom layer of milk solids. Stir in 3 tablespoons hot clarified butter into 10 cups popcorn; and refrigerate the rest for stir-frying.
Photo source: Flickr/Edward Kimber, cc by nc-nd 2.0