Recent impact simulations show that a planet’s iron core can be greatly heated by a giant impact. Here, the evolution of the coupled mantle and core after such impact heating of the core is examined using a 2D mantle model coupled to a 1D core model. A very hot core causes rapid melting of the base of the mantle and rapid core cooling, until the mantle temperature has reached the rheological transition. The hot, partially-molten material can then rise rapidly as plumes. This mechanism acts as a thermostat on core temperature, rapidly bringing it back to the pre-impact value. If the melt is iron enriched then solid-melt segregation can result in the formation of an iron-rich partially molten silicate layer above the core-mantle boundary.