We have two expressions made out of cake, almost the same as in English.
1) C’est du gâteau. Literally… it is some cake, your equivalent of It’s a piece of cake
2) La cerise sur le gâteau. Literally… the cherry on the cake, your equivalent of Icing on the cake.
You will probably find many French mentioning the cherry on the cake, don’t be surprised it’s their literal translation of the icing 😉