Couldn't they have picked something more sensible like gold, silver or obsidian?
Aztec economy was much more complicated than just cocoa beans (chocolate) as currency. They also used cotton sheets and copper axes, and a lot of bartering. Here is a good thread on Aztec economy featuring answers by /u/DownvotingCorvo and /u/400-Rabbits
In addition to the link provided by /u/grandpohbah, you might also be interested in these past discussions:
Wherein /u/historianla talks about the continued use of cacao into the the Colonial period, as well as exchange rates, and I chime in to note why cacao ended up as a mode of exchange (hint: it doesn't grow in the Central Highlands).
Which is more about general socio-economics of Aztec society.
Also, I want to reiterate, it was the beans, not processed cacao, that was used a currency. The kind of chocolate we think about today is a post-Contact invention. Mesoamericans consumed cacao as a drink. Coe and Coe's True History of Chocolate is a very accessible book on the topic if you want to learn more.