An optimized design for a composite heat sink is provided utilizing a high thermal conducting base and low thermal conducting fins. The base preferably is constructed from anisotropic graphite material, and the fins are preferably constructed from a thermally conductive plastic material. In the case of a low profile heat sink having a fin height of no greater than about 3 times that of the base, the composite construction provides superior cooling yet lighter weight as compared to a conventional all-aluminum heat sink of the same dimensions.