Power outage with flourless chocolate cake in the oven! Leave in with residual heat? Room temp and wait? Fridge out so can't keep it that cold?
Leave it in the oven. Do not open the oven. At least it's not a cake for which you want a fair bit of rise, for which you need heat. How far into the bake did the power go out? ;o)
I agree with Antonia, leave in the oven & keep closed to let as much heat as possible remain.
Do you have a BBQ grill and a pizza stone?