1 Preheat the oven to fan 170°C/Gas mark 5. Grease the inside of a 24 x 20 cm china or glass baking dish. Rinse the black beans and leave to drain. Melt the butter, if using, in a saucepan over a gentle heat, then set it aside.
2 Place the drained beans, eggs, cacao powder, 150ml of the maple syrup, the vanilla extract and coffee extract, if using, into a food processor with a pinch of salt. Pulse a few times, then blend until smooth.
3 Add the melted butter, or coconut oil, very slowly so as not to cook the eggs, while the machine is running. Taste the batter, adding more maple syrup or a few drops of stevia if needs be, then stir in most of the chopped walnuts, reserving a handful.
4 Pour the brownie mixture into the prepared dish and gently tap the baking dish on a kitchen counter to even out the mixture. Sprinkle the remaining walnuts on top and bake for 40–45 minutes, until the brownie feels firm but springy and its surface is cracked.
5 Leave to cool completely before cutting into squares. Refrigerating the brownies makes them wonderfully fudgy.
2 tins cooked black beans (drained weight 500g, or 500g homecooked beans)
230g butter or coconut oil at room temperature, plus extra for greasing
85g unsweetened cacao powder
150 - 180ml maple syrup
1 1/2 tbsp vanilla extract
1 tsp coffee extract (or use extra vanilla extract)
130 chopped walnuts (preferably crispy activated)
Optional: a few drops of stevia, to sweeten