  Color center is a promising platform for quantum technologies, but their
application is hindered by the typically random defect distribution and complex
mesoscopic environment. Employing cathodoluminescence, we demonstrate that an
ultraviolet-emitting single photon emitter can be readily activated and
controlled on-demand at the twisted interface of two hexagonal boron nitride
flakes. The brightness of the color center can be enhanced by two orders of
magnitude by altering the twist angle. Additionally, a brightness modulation of
nearly 100% of this color center is achieved by an external voltage. Our
ab-initio GW calculations suggest that the emission is correlated to nitrogen
vacancies and that a twist-induced moir\'e potential facilitates electron-hole
recombination. This mechanism is further exploited to draw nanoscale color
center patterns using electron beams.
