A human cannot by force of will [...] make her brain stop emitting heat.

Disputed! I'm pretty sure if somebody studied under controlled conditions for enough years they'd be able to induce mild hypothermia through some sort of meditative exercise.

More realistically, albeit not through willpower alone, a human could sit in a bathtub full of ice water, dose themselves with ethanol or sodium thiopental, or otherwise use external, independently-verifiable means to temporarily reduce their cognitive capacity.