Halle Berry -- Post-Preggo Knockout ... WHOA MAMA

Whatever Halle Berry's taking ... it's working like a charm -- the 47-YEAR-OLD stepped out last night for the first time since giving birth to her SECOND kid ... looking like someone half her age.

Halle was at the Acapulco Film Festival in Mexico ... but her bod was the real show.

Halle and fiance Olivier Martinez welcomed son Maceo in October ... which is crazy enough considering Halle's age ... but even crazier because the pregnancy had ZERO effect on her physique.

In fact, it might have made her even hotter ... just look at those boobs.