A heat sink, comprising: a core body, defining: a coolant plenum; sealed first and second chambers separated from one another by the coolant plenum, wherein the sealed chambers house a phase change material; and first and second coolant conduits separated from one another by the coolant plenum, wherein both coolant conduits are fluidly isolated from the sealed chambers and in fluid communication with the coolant plenum for controlling pressure within the sealed chambers while transferring heat into or out of the phase change material, and characterized in that the first coolant conduit has a wall with opposed passage and chamber surfaces, the chamber surface bounding a lateral edge of the first chamber; and wherein the second coolant conduit has a wall with opposed passage and chamber surfaces, the chamber surface bounding a lateral edge of the second chamber, and a header fluidly coupling each of the coolant plenum, first coolant conduit, and second coolant conduit with one another, wherein the header spans respective widths of the first chamber, the coolant plenum, and the second chamber. a core body, defining: a coolant plenum; sealed first and second chambers separated from one another by the coolant plenum, wherein the sealed chambers house a phase change material; and first and second coolant conduits separated from one another by the coolant plenum, wherein both coolant conduits are fluidly isolated from the sealed chambers and in fluid communication with the coolant plenum for controlling pressure within the sealed chambers while transferring heat into or out of the phase change material, and characterized in that the first coolant conduit has a wall with opposed passage and chamber surfaces, the chamber surface bounding a lateral edge of the first chamber; and wherein the second coolant conduit has a wall with opposed passage and chamber surfaces, the chamber surface bounding a lateral edge of the second chamber, and a header fluidly coupling each of the coolant plenum, first coolant conduit, and second coolant conduit with one another, wherein the header spans respective widths of the first chamber, the coolant plenum, and the second chamber. a coolant plenum; sealed first and second chambers separated from one another by the coolant plenum, wherein the sealed chambers house a phase change material; and first and second coolant conduits separated from one another by the coolant plenum, wherein both coolant conduits are fluidly isolated from the sealed chambers and in fluid communication with the coolant plenum for controlling pressure within the sealed chambers while transferring heat into or out of the phase change material, and characterized in that the first coolant conduit has a wall with opposed passage and chamber surfaces, the chamber surface bounding a lateral edge of the first chamber; and wherein the second coolant conduit has a wall with opposed passage and chamber surfaces, the chamber surface bounding a lateral edge of the second chamber, and a header fluidly coupling each of the coolant plenum, first coolant conduit, and second coolant conduit with one another, wherein the header spans respective widths of the first chamber, the coolant plenum, and the second chamber.