buttered bun, n.³ — Green’s Dictionary of Slang

buttered bun n.³

also buttered scone

[rhy. sl.]

1. (UK und.) a son.

1935

Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 2: Buttered bun: Son.

2. (bingo) the number one.

1981		P. Wright Cockney Dialect and Sl. 109: 1 = buttered scone.
1998		R. Puxley Fresh Rabbit.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.