I recently discovered the blog Peaks and penguins, by a young Canadian/US couple who lived in South Korea for some years (and maybe still do). They chronicled their explorations of the mountains there, guided by the lists of 100 top mountains by the Korean Forest Service and a commercial hiking wear/gear manufacturer. (80 mountains appear on both lists and 20 are unique to each, so there’s 120 in total, which I think they explored all.) I am half disappointed that I spent so much of the time I was in Korea not exploring mountains and a quarter excited and a quarter daunted that there’s so much for me to do when I go there again (when, when, when?). And that’s just the mountains, not all the other things to do.
One of their early expeditions nearly ended badly: the weather changed, they were short on warm clothing and other provisions, and they lost their way. Fortunately they encountered a Korean hiking group who warmed them up and pointed them in the right direction. They wrote:
Our descent was cake compared to our trials on the ridge.
Was cake, not was a piece of cake, which is an established idiom.
I haven’t been able to find any equivalent use of “was cake” (in quotation marks for exact match). There are sentences like When was cake first made?, We heard/were told there was cake and And then there was cake. But I can’t say that those bloggers are wrong; it’s very clear what they mean and is a natural shortening (<haha) of the idiom. Maybe people say it or write it in places Google can’t find.
According to Google Ngrams, is/was a piece of cake rose in usage in the mid-1970s. Without context, it’s impossible to tell how many occurrences before and after then were literal usages of the phrase, and how many were idiomatic. I had always assumed that it is/was a piece of cake was, in turn, a shortening of it is/was as easy as eating a piece of cake, but Ngrams shows no particular usage of as easy as eating.
Is/was a piece of cake seems to be used/usable in singular forms: My homework was a piece of cake, ?My exams were pieces of cake, My exams were a piece of cake.