For the safe and efficient operation of concentrator photovoltaic cells and electronic chips, low and uniform temperature should be attained. Therefore, the prime focus of this study is to design the optimal headers and to evaluate the performance of a monolithic double-layer microchannel heat sink (MDL-MCHS) operating under forced convective boiling conditions. The designed and fabricated heat sink was proved to attain a uniform temperature distribution over the entire surface of the MCHS heated wall, in a narrow temperature range around the coolant boiling point. The designs of the MCHS inlet and outlet headers were computationally optimized to avoid flow maldistribution over 10 parallel channels in each layer. Subsequently, an MDL-MCHS with an optimized header was fabricated using a metal 3D printer, and its thermal characteristics were experimentally evaluated in counterflow and parallel-flow operations under single-phase liquid flow and forced convective boiling conditions. The supplied heat flux was varied from 1.0 to 9.2 kW/m(2). Ethanol and acetone with a boiling point of 78.4 degrees C or 56 degrees C were identically fed into each layer in a flowrate ((V) over dot) range of 15-400 ml/h. At 9.2 kW/m(2) (11.5 suns), the counterflow operation of forced convective boiling attained temperature uniformity below 1.6 degrees C and 1.8 degrees C in the (V) over dot range of 25-100 ml/h for ethanol and 50-300 ml/h for acetone, respectively. The resultant wall temperature was nearly identical with the boiling point of operated coolant.