Butter is a Human condiment, used in and on many varieties of food.
When Tom Paris recreated the Palace Theater, he replicated some popcorn for his date with B'Elanna Torres. Later when the crew was invited to a showing of Attack of the Lobster People, Neelix would stand by the entrance, offering popcorn to people as they enter "buttered" or "unbuttered". (VOY: "Repression")