I used this recipe from Whole Foods. Changes I made: subbing coconut oil for butter, going just shy of 1/2 cup for the sugar, omitting the walnuts, increasing chocolate chips to just under 1 cup (mixed in the batter and sprinkled on top). Overall, not bad for a brownie made with black beans. Slightly grainy in texture, and a little too cakey for my taste. Plus, they stuck to the bottom of the pan.
Next time, I will under-bake, and consider swapping mashed banana for the sugar. Perhaps will omit eggs and use a ground flaxseed slurry to make it vegan (with the coconut oil). Needs work.