You take a straw . . . shove one end down into the popcorn . . . and line it up with the butter dispenser. So that way, the liquid butter goes down the straw, and popcorn at the bottom gets some too. You could even put pockets of butter all over the place. (Here's the video.)
Obviously it has to be a theater where you pour your own butter. But it seems pretty legit. We also found a Reddit post from a few years ago that says it works really well. (For theaters that don't have self-serve butter, just ask for your butter "layered," and they'll put some in the middle.)