Employing universality arguments, we construct, based on the critical 3d Ising effective action [M.~M. Tsypin, Phys. Rev. Lett. {\bf 73}, 2015 (1994)], the {\it Ising-QCD partition function} for the description of the net baryon density in the immediate neighbourhood of the QCD critical endpoint, remnant of the chiral phase transition. We demonstrate that the constructed partition function satisfies all the requirements of scaling theory and universality, not only for an infinite system, but also in the finite size scaling regime. Furthermore, we use the Ising -QCD partition function to obtain predictions relevant to measurements in the search for the QCD critical point in contemporary and future experiments with colliding nuclei.