Preheat the oven to 325*F, and grease a small (8x8 or 9x9) pan. Break up the unsweetened chocolate, and combine it with the fat (butter, coconut oil or shortening). Heat the mixture on low power in the microwave or in a double boiler, stirring often, until it's melted and combined.
Meanwhile, put the black beans, agave, vanilla, espresso powder, and salt in a food processor. Add the chocolate mixture, and process 2-3 minutes - until the black beans are obliterated and the mixture is well-combined. Add almond flour (or coconut flour), flax meal, and baking soda. Process two minutes more. Fold in nuts & chocolate chips, if using.
Pour the mixture into the greased baking pan. Bake for 40-50 minutes - until the top is set, but be careful about burning the edges. Allow to cool for at least an hour; otherwise, the brownies may be too gooey to slice.
Store covered on the counter, or for a firmer brownie, in the refrigerator.