frosting/icing on the cake, the

icing on the cake

Fig. an extra enhancement. Oh, wow! A tank full of gas in my new car. That's icing on the cake! Your coming home for a few days was the icing on the cake.
See also: cake, icing, on
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

icing on the cake

Also, frosting on the cake. An additional benefit to something already good. For example, All these letters of congratulation are icing on the cake, or After that beautiful sunrise, the rainbow is just frosting on the cake. This metaphoric expression alludes to the sweet creamy coating used to enhance a cake. [Mid-1900s]
See also: cake, icing, on
The American Heritage® Dictionary of Idioms by Christine Ammer.

icing on the cake

n. an extra enhancement. Oh, wow! A tank full of gas in my new car. That’s icing on the cake!
See also: cake, icing, on
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions

icing on the cake

An additional benefit to something already good.
See also: cake, icing, on
American Heritage® Dictionary of the English Language, Fifth Edition.

frosting/icing on the cake, the

An extra advantage or additional benefit. This term refers to the sweet creamy topping of a cake and has been transferred since the mid-1900s. A book review in The Listener used it: “All this theology is icing on the cake” (April 3, 1969; cited by the OED).
See also: frosting, icing, on
The Dictionary of Clichés by Christine Ammer
See also:
  • That takes the cake!
  • a piece of cake
  • be a piece of cake
  • piece of cake
  • piece of cake, it's a
  • whoopie cake
  • whoopie cakes
  • yellow cake
  • easy as A, B, C
  • berries