It’s as easy as falling off a piece of cake

English: Wedding cake (Photo credit: Wikipedia)

This is a mash up of “it’s as easy as falling off a log”, “easy as pie”, and “it’s a piece of cake”, all meaning something very easy.   This is a great example of a congruent malaphor, when two or more root expressions have the same or similar meaning.  These kinds of malaphors are almost always understood by the listener because the idioms express the same thought.