  In this paper we provide a general framework based on $\delta N$ formalism to
estimate the cosmological observables pertaining to the cosmic microwave
background radiation for non-separable potentials, and for generic \emph{end of
inflation} boundary conditions. We provide analytical and numerical solutions
to the relevant observables by decomposing the cosmological perturbations along
the curvature and the isocurvature directions, \emph{instead of adiabatic and
entropy directions}. We then study under what conditions large bi-spectrum and
tri-spectrum can be generated through phase transition which ends inflation. In
an illustrative example, we show that large $f_{NL}\sim {\cal O}(80)$ and
$\tau_{NL}\sim {\cal O}(20000)$ can be obtained for the case of separable and
non-separable inflationary potentials.
