Phase change materials (PCMs) are effective at storing thermal energy and are attractive for use in electronics to smooth temperature peaks during periods of high demand; however, the use of PCMs has been somewhat limited due to the poor thermal properties of the materials. Here, we propose a design for a tunable composite PCM heat sink for passive thermal management in electronic systems and develop an improved test platform to directly compare performance between different designs and PCMs. The composite design leverages high conductivity pathways, which are machined into aluminum heat sinks, and back-filled with PCMs. Two package sizes are considered with several internal fin structures. All designs are evaluated using a test platform with realistic power profiles, controlled interfacial loading, and in situ temperature measurement. The composite PCM heat sinks are benchmarked against solid aluminum packages of the same size. This study focuses on three commercially available PCMs. Performance is evaluated based on (1) the time it takes the test heater chip below each composite PCM package to reach the cut-off temperature of 95 °C and (2) the period of a full melt-regeneration cycle. A range of heat fluxes are considered in this study spanning 6.8–14.5 W cm−2. The isokite design with PlusICE S70 extends the time to reach 95 °C by 36.2% when compared to the solid package, while weighing 17.3% less, making it advantageous for mobile devices.