I've been cooking this recipe for years. I've based it on some recipes for Costa Rican black beens I'd been screwing around with since visiting. There, they add what looks like an obscene amount of garlic, and let it simmer with the beans until it's lost its bite, and mellowed into something warmer and deeper that pervades the entirety of the dish.
The guacamole is a recipe I learned from an friend's mother, who'd long-since despaired of either of her girls ever taking an interest in cooking. It's genius is in its simplicity, but it takes some screwing with the ratios to get it right.
The brown sugar serves dual roles here: it both balances out the heat, and maintains the calcium structure of the beans (according to Harold McGee, anyway!). Use a light hand with it!
While making the chili as-is keeps it vegan (depending on your vegan friends' notion on brown sugar), I like to top it with a little sharp cheddar, along with the necessary fat spoonful of guacamole.
Serve with warmed tortillas, guacamole, sour cream (vegan, if ya like), scallions, cheddar cheese, whatever. I like to serve mine over Mexican-style brown rice.
DISCLAIMER: everything here is very much guess-and check. —Ian