We stack-engineer an optically gated thermionic valve which acts as a thermometer for hot electronic charge carriers. Through pulsed photoexcitation of encapsulated graphene-boron nitride-graphene heterostructure devices, we trigger picosecond charge carrier transit through an ultrathin BN barrier. The resultant interlayer photoconductance exhibits extraordinary enhancement near the charge neutrality (Dirac) point of graphene. We attribute this to ultrafast thermionic cascade arising from an elevated electronic temperature from the rapidly thermalizing electrons and holes. The electronic temperature was found to peak at the Dirac point and reaches well above 2000 K. This Dirac point thermionic cascade can also be dramatically quenched via electrical control at low temperatures, which indicates unconventional-yet highly efficient- cooling pathways that serve as a hallmark of Dirac electron-hole plasma in graphene.