The problem here is partially timing and the second whipping; it's better to fold-in dry ingredients rather than whip into an already formed meringue.

 The cocoa added to the whipped egg white doesn't help stabilize it, so whipping it again once it's added will cause the bubble-structure to collapse rapidly.  Delicately folding in the cocoa until it is just mixed into the meringue should work.  

This is also how one adds almond flower to meringue to get Macarons.