Isn't the idea of a Boltzmann Brain not that it poofs out of existence, but that there is no way to prove that you are not a Boltzmann Brain, and all of your perceived experiences could be confabulation? In such a scenario the Brain needn't poof out of existence; merely continuing to exist for but a moment would belie the true nature of their consciousness, as the brain quickly suffocates and dies, losing its contents.