Circular dichroism (CD) is generally observed in the optically active chiral molecules that originate from macroscopic electric and magnetic dipoles, which is usually quite small. In solid states, the so-called valley CD may arise microscopically from interband transitions between two chiral electronic valley bands of nonzero Berry curvatures at a given k point. However, generally, two sets of K and K′ valleys coexist in the Brillouin zone with opposite chiral selectivities, so that the net CD is zero for the whole material. Here, we demonstrate a giant CD originating from photoexcitation between two chiral Chern flat bands of opposite Chern numbers, namely, the enantiomorphic flat Chern bands. The dissymmetry factor g of such flat CD can reach the theoretical maximum value of 2 with the optimal spin-orbit coupling strength. Based on first-principles calculations, we identify that the Li intercalated bilayer π-conjugated nickel-bis(dithiolene) hosts a set of yin-yang kagome bands with an estimated large g=0.74 under magnetic field. Furthermore, based on the flat-CD mechanism, we propose two flatband devices of topological photodetectors and circularly polarized lasers.