Reduce the hot taste in a Cambodian curry
If it is about keeping authenticity, more coconut milk would be the answer. If you just want a toned down but well tasting curry, any emulsified fat - coconut milk, cream, yoghurt - will do, as long as you add it carefully in a way that doesn't make it split and become grainy (so whisk it, add it carefully with the heat turned off). Cream and/or yoghurt would be the authentic addition for a north indian curry.