Chilli con carne - chocolate, cocoa powder or nothing?
The origin of chili con carne is actually from the American southwest, not Mexico. It's tex-mex, not mex. 

There are many variations on chili con carne, however if you go back to the earliest known recipes cocoa or chocolate are not on them. It's pretty basic, flavored mostly by cumin and chili. So if you want to make the most authentic chili to the origin then skip the cocoa. 

However, there are many good recipes out there which have chocolate or cocoa in them and they are as valid as any other, so it all comes down to personal taste. I add unsweetened cocoa powder to my chili because the bitterness balances out the acidity of the canned tomatoes, and I like the flavor combinations you get. The umami from the chocolate enhances the flavor, and I like the way it hits the tongue, it gives it more punch. 

Once I made a chili and split it into two, with one I added some cocoa powder, otherwise the chilis were identical. I served both and asked people which they liked better and the vast majority preferred the one with cocoa in it. You can also add a tsp or two of instance coffee instead although I don't think it's as good. Adding sweetened dark chocolate instead of cocoa powder works just as well, there's no much difference except the chocolate adds a bit of sweetness. The main benefit of using chocolate rather than cocoa powder is that you can try different varieties, however as it's just an undertone you aren't likely to notice. 

So it's all down to taste.