The phrase ‘Icing on the Cake’ refers to something positive that enhances a situation that’s already good.
Example of Use: “She was delighted she was accepted to her new job, but having her best friend as her boss was just the icing on the cake.”
The first recipe for confectionery icing was published in 1769 in Elizabeth Raffeld’s 'The Experienced English Housekeeper', although it’s likely that icing was used sometime before this. The first cakes, though, were made in Neolithic times, and were often quite similar to bread. It’s safe to say that the first time anyone used the idiom 'Icing on the cake' was sometime after 1769, when cakes became much more enjoyable thanks to the addition of fancy toppings.