Butter is a Human condiment made from cream or milk, used in many foods.
The crew assigned to the day shift on Deep Space 9 in 2373 enjoyed a short stack of pancakes, dripping with butter, with a side of sausage and pineapple for their breakfast. (DS9: "The Assignment")
Mashed potatoes with butter was a favored dish of Samantha Wildman, who introduced it to Kes. Kes ate six bowls of it when she underwent premature elogium as a result of space-dwelling lifeforms in 2371. Neelix tasted it and pronounced it "awful," although this was because Kes had added a container of nitrogenated soil to the recipe, as her body was lacking certain minerals present in soil at that time. (VOY: "Elogium")
When Tom Paris recreated the Palace Theater on the holodeck in 2377, 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")