Fast Photon Counting Optical Astronomy (FPC-OA) is tailored to studying sub-second optical variability in astrophysical sources by means of photon counting instrumentation and techniques. AQUEYE+ and IQUEYE are two instruments built to this purpose, and to bring Quantum Optics concepts into the domain of optical Astronomy. The FPC-OA project is under continually scientifical and technological development, and is producing cutting-edge results on optical pulsars and the search for optical flashes in Fast Radio Bursts and magnetars. Observations are carried out within the framework of multi-instrument/multi-wavelength campaigns and in synergy with facilities operating at other wavelengths. With this instrumentation we also performed the first pilot experiment of photon counting Stellar Intensity Interferometry on a km baseline. The goal of this experiment was to measure the correlation of star-light counting photon coincidences in post-processing by means of a single photon software correlator and exploiting entirely the quantum properties of light. I will also report on the ongoing realization of new photon counting instrumentation that will be installed on the ASTRI Mini-Array of Cherenkov telescopes to perform optical Stellar Intensity Interferometry observations, with the extraordinary goal of achieving an angular resolution of ~100 microarcsec. At this level of resolution it will be possible to reveal unprecedented details on the surface and of the environment surrounding bright stars on the sky.