Pre-heat oven to 180 degrees and grease a 9″ x 12″ rectangle baking dish with coconut oil.
Mix the dry ingredients together in a separate bowl (flour, cacao powder, sugar baking powder and baking soda.
Chop the chocolate into a heat-proof bowl, add the butter and place in the microwave for 15 seconds intervals and stir until melted. Once melted (but not hot), whisk in the egg.
Pour the wet mixture into the dry mixture, then whisk it all together. Immediately add the hot water and mix very well, then pour the batter into the prepared dish.
Place in the preheated oven and bake for 45 to 50 minutes or until a skewer inserted comes out clean. Set aside to cool.
Make the mixed nut frosting:
Put the mixed nuts butter, icing sugar and mascarpone cream in a food processor/blender and blitz until smooth.
Pour this mixture over the brownies that should have cooled down and spread it over the top with a spatula.
Slice the banana and place one slice at a time all over the icing. Then finish by grating a small piece of dark chocolate all over.
Freeze for two hours and using a bread knife, slice into the brownie to make square bars. serve cold.