  The mass of heavy quarks, such as charm and bottom, plays an important role
in the formation of parton showers. This effect is apparently not well
understood when parton showers evolve in a strongly interacting quark-gluon
plasma. We propose a new experimental measurement in relativistic heavy ion
collisions, based on a two-prong subjet structure inside a reconstructed heavy
flavor jet, which can place stringent constraints on the mass dependence of
in-medium splitting functions. We identify the region of jet transverse momenta
where parton mass effects are leading and predict a unique reversal of the mass
hierarchy of jet quenching effects in heavy ion relative to proton collisions.
Namely, the momentum sharing distribution of prompt $b$-tagged jets is more
strongly modified in comparison to the one for light jets. Our work is useful
in guiding experimental efforts at the Large Hadron Collider and the
Relativistic Heavy Ion Collider in the near future.
