the.com/piece of cake

means Something so easy to do that it requires almost no effort at all.

from An American phrase first recorded in the 1930s, with the earliest known print use crediting poet Ogden Nash. Why cake, specifically? Cake has long stood for something pleasant and effortlessly enjoyed — think of the older phrase 'cakewalk,' a 19th-century dance contest where the prize was, fittingly, a cake, and where the winning performance looked easy and showy rather than strenuous. 'Piece of cake' picked up that same association: cake as the easy, pleasant reward rather than the hard-won one.