The final bit of icing on the cake you offer someone who has just made an obvious (to you at least) unreasonable demand:

"I'd like a full report in triplicate on my desk by lunchtime"
"Okay then, and would you like that with a cherry on top?"

Probable origin is from the delectable ice cream desert, a knickerbocker glory.

Log in or register to write something here or to contact authors.