How to best store cocoa powder?
Store cocoa powder in a dark, cool, dry place, sealed against vermin.  Dark and cool both slow the process by which volatiles (i.e., flavor) degrade.  That said, don't keep it in the fridge or freezer unless sealed airtight, because both types of chill-chests are relatively humid environments.  Humidity promotes mold, even on cocoa.

By the way, for future reference:  When buying cocoa powder, note the manufacturer's suggested use-by date.  Cocoa powder should last about three years, properly stored.  If the use-by date is much less than that, look for another container.