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) Fish fingers and custard became an important part of the Doctor's friendship with Rory and Amy.
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)