You should just use plain butter or butterfat. Butterfat is what's used in (good) theaters and is the closest to real butter. Butterfat doesn't have colors or milkfat, which may burn in the warmers after awhile, not to mention start to stink up the place. So it's more practical than store butter. It tastes slightly different from store butter, however.
If you really want the true no-compromise theater taste you can make your own butterfat by just heating regular butter and skimming off the foamy stuff.
The other keys to movie theater popcorn are the oil and salt. We used red palm oil but coconut oil is fine. We added enough popcorn salt to the bucket during popping to give it a very lightly salted flavor.
Thinking about it, you could probably use some strained bacon fat to make bacon-flavored popcorn, and/or mix some bacon fat into the butter.