Butter was a spread made from milk, particularly the fat it contained. As a child, Amy Pond served the Eleventh Doctor bread and butter. He didn't like it and threw it away, saying, "And stay out!" (TV: The Eleventh Hour)
The Tenth Doctor helped Layla make butter. (AUDIO: Dead Air)
Tibetans made tea with yak butter in it to fortify them from the cold. (COMIC: Yonder... The Yeti)
Ruth made buttered toast for breakfast on her birthday. (TV: Fugitive of the Judoon)