Preheat the oven to 350°F. Place the beans in a sieve and rinse well with cold water. Set the sieve over a bowl or in a sink and let the beans drain 30 minutes.
Place the butter and chopped chocolate in a small bowl and zap in the microwave, 30 to 60, just until the butter and chocolate have melted. Mix to combine the two and then set aside.
Preheat the oven to 350°F. Cut an 8” by 12” piece of parchment paper to fit the bottom and up 2 of the 4 sides of an 8” square baking pan. (The parchment paper extending the sides of the pan will later be used as handles to lift the square out of the pan once baked.)
Place the beans, melted chocolate mixture, sugars, vanilla and cinnamon in a food processor. Pulse until very smooth and evenly blended. Transfer the mixture to a medium bowl. Fold in the flour until just combined. Now mix in the chocolate chips and walnuts.
Evenly spread the batter into the prepared pan. Bake for 26 to 28 minutes, or until the brownie is set and springs back when gently touched in the centre.
Cool the brownie on a baking rack, and then cover and refrigerate to firm up. Cut around the edges of the brownies, and then lift it out of the pan. Dust with icing sugar or cocoa, cut into squares, and serve.
Note: 1 (540 mL) can of black beans contains about 2 cups of beans, when drained. Save the ones you won’t need in this recipe for another use, such as adding them to steamed rice, a salad or a soup.