Butter

Butter was a spread made from milk, particularly the fat it contained. Types of butter included yak butter (COMIC: Yonder... The Yeti) and cocoa butter. (PROSE: Our Flavours)

One of the three universal constants was toast always falling butter-side down, along with Planck's quantum of action and the reaction of someone entering the TARDIS for the first time. (PROSE: The Secret in Vault 13) Indeed, on 11 June 2022, the reader of a guide for time travellers dropped their breakfast and the toast landed butter-side down. The author noted this as a fixed point in time and warned the reader to not attempt to avert it or they could destroy the universe. (PROSE: )

Use of butter
Bread and butter was the combination of bread with butter; (TV: The Eleventh Hour) similarly, buttered toast could be created by spreading butter onto toast. (TV: Fugitive of the Judoon) Butter was also seemingly used in chip sandwiches. (TV: Revenge of the Slitheen) Tea could be made with yak butter. (COMIC: Yonder... The Yeti)

Butter and cocoa butter were also listed as ingredients of Millingdale's "Death By Chocolate" ice cream on their website's "Our Flavours" page. (PROSE: Our Flavours)

History
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)

Learning that a Slitheen feared Clyde Langer's cold chip sandwich, Maria Jackson briefly suggested the Slitheen were allergic to butter before Sarah Jane Smith recalled seeing a Slitheen eating a sandwich. (TV: Revenge of the Slitheen)

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)