What is the origin of the idiom 'the icing on the cake'?

Icing on the cake would seem to be a uniquely English aphorism. The etymology of "icing on the cake" is probably as simple as it appears: something sweeter on top of something already sweet. For example, "I got the promotion, they raised my salary and they gave me a corner office." The "cake" in this case would be the promotion. The remainder would be the "icing on the cake."