  A two-dimensional topological crystalline insulator (TCI) with a single unit
cell (u.c.) thickness is demonstrated here. To that end, one first shows that
tetragonal ($C_4$ in-plane) symmetry is not a necessary condition for the
creation of zero-energy metallic surface states on TCI slabs of
finite-thicknesses, because zero-energy states persist even as all the in-plane
rotational symmetries--furnishing topological protection--are completely
removed. In other words, zero-energy levels on the model are not due to (nor
are they protected by) topology. Furthermore, effective twofold energy
degeneracies taking place at few discrete $k-$points away from zero energy in
the bulk Hamiltonian--that are topologically protected--persist at the
u.c.~thickness limit. The chiral nature of the bulk TCI Hamiltonian permits
creating a $2\times 2$ square Hamiltonian, whose topological properties
remarkably hold invariant at both the bulk and at the single u.c.~thickness
limits. The identical topological characterization for bulk and u.c.-thick
phases is further guaranteed by a calculation involving Pfaffians. This way, a
two-atom-thick TCI is deployed hereby, in a demonstration of a topological
phase that holds both in the bulk, and in two dimensions.

Mesoscale and Nanoscale Physics