A heat sink for directly cooling at least one electronic device package is provided. The electronic device package has an upper contact surface and a lower contact surface. The heat sink comprises a cooling piece formed of at least one thermally conductive material. The cooling piece defines multiple inlet manifolds configured to receive a coolant and multiple outlet manifolds configured to exhaust the coolant. The inlet and outlet manifolds are interleaved. The cooling piece further defines multiple millichannels configured to receive the coolant from the inlet manifolds and to deliver the coolant to the outlet manifolds. The millichannels and inlet and outlet manifolds are further configured to directly cool one of the upper and lower contact surface of the electronic device package by direct contact with the coolant, such that the heat sink comprises an integral heat sink.