Heat engines are an integral part of our daily lives. They power cars or produce electricity by converting heat into work. Increasing their efficiency is very difficult and only marginal improvements have been achieved over the last decades. Thus, to reach the ambitious climate goals, it is necessary to go beyond conventional technologies. Atom-sized systems where quantum mechanical effects come into play could enable this: theory predicts that their efficiency can be boosted beyond the classical limits imposed by thermodynamics. However, so far, this has not been tested in practice due to a lack of suitable model systems. I propose to build a molecular heat engine of only a few atoms in size, with such high control over its structure and properties that these predictions can finally be tested. The engine's quantum properties will be robust at experimentally accessible temperatures, its coupling to the environment will be controllable, and electrical transport through it will be quantum coherent. I seek to exploit the full gamut of their physical properties to boost efficiency, including spin entropy and vibrational coupling. Practically, I will 1) implement a scanning probe setup into a dilution refrigerator, 2) fabricate single-molecule junctions with micro-heaters and ultra-sensitive superconducting thermometers, and 3) perform and interpret caloric experiments on single molecules at unprecedented precision. The results will teach us about the fundamental properties of atom-scale quantum systems and heat flowing through single molecules. It will inspire new ways to increase the performance of thermoelectric applications such as waste heat harvesters, nanoscale spot-cooling devices, or even thermal rectifiers and transistors. I am one of the forerunners in molecular thermoelectrics, with extensive hands-on experience in material sciences, nanotechnology, and mesoscopic physics. This multidisciplinary background is needed to make this ambitious project a success.