I don't think it is used that way. Here is an Italian text explaining the meaning of "piece of cake" as "qualcosa di molto facile", so they probably don't have the same saying: http://blog.babbel.com/it/lamericano-colloquiale-facile-come-pezzo-torta/ .
Interesting enough, "pezzo di torta" doesn't seem to be your first choice of describing a slice of cake, either, that would be "fetta di torta". http://www.wordreference.com/iten/pezzo%20di%20torta vs http://www.wordreference.com/iten/fetta%20di%20torta
Here is Reverso Context's take on it. It seems that "pezzo di torta" would be taken literally to mean a slice of cake, and what English speakers refer to as a "piece of cake' meaning something easy to do would be something like gioco da ragazzi or perhaps una passeggiata