Well, it depends on how authentic you want to be...and how involved.
First option, I mentioned in the comments - there are chocolate elixirs available for purchase at a company called kakawa chocolates. From what I've read, these are as historically authentic as one can get, they use archaeological research and written records from the conquistadors. Not a lot of work on your end, but it should do the job with only an outlay of cash-money.
Second option, you can look up recipes, if you can figure out good search terms. Since I'd already known kakawa chocolates, searching for a copycat recipe found this one: a mayan chile elixer that looks reasonable, using unsweetened chocolate, and honey instead of processed sugar. Searching for something like "authentic chocolate drink aztec" (or mayan, or omitting drink for solid versions) may find some recipes. One at allrecipes seems simple enough, another at cdkitchen has similar ingredients but seems to have processes looking to replicate authentic texture, this other one from kitchenreport has more handwavy amounts but a much more rustic process if you want to start from near-scratch, and this last one from yucatanadventure looks like it may well be an authentic, researched recipe. Searching for xoacoatl, which is apparently one of the original names, may also yield some good recipes - one here at uncle phaedrus, here's another one at almoustine, they both start from raw beans, but if that's a bit much this one from meltingmug looks a bit easier.
Third option is to do the research yourself and get really historical recipes. A couple places mention the Spanish conquistadors written records, as a good place to start from. Historical recipes can be pretty tricky, its best to find someone willing to translate - especially since they assume familiarity with the basic cooking techniques instead of spelling out all the steps, which makes it a bit hard for those starting from scratch, and some of the ingredients or processes they take for granted are quite different now, and doubly especially since the beginning language isn't english, which may well make it harder to track down good translations.
You'd want to look at historical and cultural sources first and foremost, to get at the original accounts, since most recipe collections tend not to include the source material, just a summary for interest. This will take a lot of work and research, but it can be fun - I've an interest in medieval recipes, so I'm a bit familiar reading about the difficulty (and fun) of working with accurate historical documents, and dealing with cultural and temporal translations.