If the quick definitions that "onelook" offers are anything to go by:

Quick definitions (cacao)
# noun: tropical American tree producing cacao beans

Quick definitions (cocoa)
# noun: powder of ground roasted cocao beans with most of the fat removed
# noun: a beverage made from cocoa powder and milk and sugar; usually drunk hot

Sorry, don't have the time to check anything else. Seems they are pretty interchangeable.

ist doch egal sag beides

cocoa is the whole thing, plant, natural cocoa

cocao is basically cocoa with the lovely cocoa butter removed, which makes real cocoa really tasty...

my own special recipe for real cocoa: one cocoa ball, melt briefly in a mug in the microwave, add water and heat - needs nothing - in its own way as good as any espresso...

as you can see you melt the cocoa - so much butter is in it, which is removed by chocolate companies to use for other purposes, like cosmetics etc. and other fats and stuff is substituted.

cocao powder is very much the poor relation and has little to do with the fab original...