Does cocoa powder need a hechsher?

Answer: No, if plain (Star-K, cRc).

Reason: Plain cocoa powder (not cocoa mix) is fine.

Source:

• Star-K: its list of products that need no hechsher includes plain cocoa powder (not cocoa mix), provided there are no additives and it is not a product of Israel.
• cRc: plain cocoa is acceptable without hashgachah if there are no questionable additives; cocoa butter requires certification.

Related questions:

• Does sugar need a hechsher? No (Star-K, cRc).
• Do baking soda and baking powder need a hechsher? No (Star-K, cRc).

References: Star-K, “Approved Without A Hechsher” list · cRc (askcrc.org), “Cocoa (plain)”