Air cooling systems are currently the most popular and least expensive solutions to maintain a safe temperature in electronic devices. Heat sinks have been widely used in this area, allowing for an increase in the effective heat transfer surface area. The main objective of this study was to optimise the shape of the heat sink geometric model using the Adjoint Solver technique. The optimised shape in the context of minimal temperature value behind the heat sink is proposed. The effect of radiation and trapezoidal fin shape on the maximum temperature in the cooling system is also investigated. Simulation studies were performed in Ansys Fluent software using the Reynolds—averaged Navier−Stokes technique. As a result of the simulation, it turned out that not taking into account the radiation leads to an overestimation of temperatures in the system—even by 14 ∘C. It was found that as the angle and height of the fins increases, the temperature value behind the heat sink decreases and the heat source temperature increases. The best design in the context of minimal temperature value behind the heat sink from all analysed cases is obtained for heat sink with deformed fins according to iteration 14. The temperature reduction behind the heat sink by as much as 25 ∘C, with minor changes in heat source temperature, has been achieved.