“Icing on the cake”

“The icing on the cake” means something great adding on to something that is already good (such as the cake). The expression “icing on the cake” can also refer to something terribly bad on top of other bad things.
 
“He’s merely an ornamental part of the business, like the icing on the cake” was cited in print in 1886. “The afternoon session was the climax, the ‘icing’ on the cake” was cited in print in 1896.
 
   
Wiktionary: the icing on the cake
Noun
the icing on the cake
1. (idiomatic) Something wonderful at the end of something good.
   
(Oxford English Dictionary)
icing, n.
orig. U.S. the icing on the cake and variants: an additional benefit to something that is already good; (also) an attractive but inessential enhancement. Now also ironic.
1896 News & Observer (Raleigh, N. Carolina) 7 Apr. 5/3   The afternoon session was the climax, the ‘icing’ on the cake.
1918 Ogden (Utah) Standard 4 Mar. 5/1   To a man love is merely the icing on the cake of life, but to a woman love is the bread of life, and she starves and perishes without it.
1950 Billboard 4 Feb. 54/3   Some fine moppet actors have already been on view this season, but it takes 12-year-old Johnny Stewart..to put the icing on the juvenile performance cake.
   
Google Books
19 June 1886, All the Year Round, pg. 428, col. 2:
He’s merely an ornamental part of the business, like the icing on the cake.
 
20 April 1913, The Idaho Statesman (Boise, ID), “Intimate Stories of the Big Leaguers” by John J. McGraw, pg. 13, col. 1:
The world’s series has always struck me as being kind of the froth on the season, the icing on the cake.
 
OCLC WorldCat record
He puts the icing on the cake
Author: Pearl Gibbs
Publisher: Washington, D.C. Christian Life 1969.
Edition/Format:   Book : English
 
OCLC WorldCat record
It was the icing on the cake
Author: Jane Addison Oakley
Publisher: New York : Vantage Press, 1982.
Edition/Format:   Book : Biography : English
 
OCLC WorldCat record
Fringe benefits. More than just icing on the cake.
Author: MD Mattera
Edition/Format:   Article : English
Publication: RN 1985 Apr; 48(4): 20-5
Database: From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.