Custard

Custard was a yellow dairy product which was consumed on Earth.

Shortly after his regeneration, a hungry Eleventh Doctor couldn't find any food palatable except custard with fish fingers dipped into it, a dish he called "fish custard". (TV: The Eleventh Hour)

Whilst on the 'Crooked World' the Eighth Doctor deliberately imagined (and therefore created) a custard pie gun. (PROSE: The Crooked World)

When repositioning his ship close to the Gateway, Rorvik ordered that "I want a landing that wouldn't ripple the skin off a custard". (TV: Warriors' Gate)

When companion Amy Pond asked the Eleventh Doctor to trust her, he asked her to swear to him on something that matters. After some thought, she smiled and said "Fish fingers and custard". (TV: The Impossible Astronaut)

Later, a TARDIS voice interface that interacted with the Eleventh Doctor in the form of Amelia also referenced "fish fingers and custard". (TV: Let's Kill Hitler) These incidents showed how much the Doctor and Amy saw the dish as a symbol of their friendship.