This is a perfect congruent conflation of “easy as pie” and “it’s a piece of cake”, both meaning a very easy task or accomplishment. Cakes and pies seem to be easily muddled in one’s mind, as can be seen in earlier posts containing pies and cakes (see, e.g, https://malaphors.com/2013/01/01/its-as-easy-as-falling-off-a-piece-of-cake/ and https://malaphors.com/2016/08/10/the-man-is-a-nut-cake/). “It’s a cakewalk” might also be in the mix, again meaning something easy. A tip of the hat to John Kooser for uttering this one and immediately sending it in! (malaphor rule #1 is write it down immediately after hearing or saying the malaphor as you will quickly forget it).