Pre-heat the oven to 170ºC/150ºC fan/325ºF/gas 3. Lightly grease and line a 20cm/8in square baking tin.
Purėe the beans in a food processor.
Add the oil, eggs, cocoa powder, sugar, vanilla and coffee, and blend until very smooth.
Add the flour, baking powder and salt, then pulse a few more times to combine.
Stir in the chopped chocolate and half of the chopped walnuts.
Pour he batter into the prepared pan and spread out to level, then top with the remaining walnuts.
Bake in the oven for 15-20 minutes, or until the edges begin to pull away from the sides and the surface begins to look dry.
Leave to cool completely in the pan before turning out and cutting into squares to serve.