XYZ is the icing on the cake (wäre das I-Tüpfelchen - zu 'abgerundet' fällt mir gerade nix ein)

http://www.thefreedictionary.com/round+off

OAED:
round sth off (with sth)
1 (NAmE also round sthout) to finish an activity or complete sth in a good or suitable way: She rounded off the tour with a concert at Carnegie Hall.