Providing an additional degree of freedom for binary information encoding and nonreciprocal information transmission, chiral single photons have become a new research frontier in quantum optics. Without using complex external conditions (e.g., magnetic field, low temperature), coupling emitters to chiral optical antennas has become a promising strategy to efficiently convert single photons from linear to circular polarization states. For ideal chiral single-photon sources, essential properties such as giant Purcell factor, large degree of circular polarization (DCP), and high collection efficiency are highly demanded. Herein, to meet these combined requirements, we propose an emitter-coupled nanobar-on-mirror antenna platform with significant local chirality acquired from the broken symmetry, as well as the giant Purcell factor owing to its ultrasmall mode volume. An emitter embedded at the corner in the gap exhibits above 3 orders of magnitude enhancement of the chiral spontaneous emission with more than 80% collection efficiency, along with up to 70% DCP. Compatible with a myriad of nanoscale quantum emitters (e.g. transition metal dichalcogenides, color centers, quantum dots, etc.), this platform, not only manifests the potential for realizing ultrafast chiral single-photon generator towards GHz and THz operation speed but also provides versatile testbeds for investigating chiral light-matter interaction at the single-quantum level.