For the brownie, preheat the oven to 170C/325F/Gas 3. Grease a 23cm/9in square cake tin.
Melt the chocolate and butter together in a saucepan over a low heat. Remove the pan from the heat and set aside to cool slightly.
Whisk the eggs until thick, then gradually beat in the sugar until glossy.
Beat in the melted chocolate mixture, then gently fold in the chestnut flour, cocoa and baking powder and salt.
Fold in the fudge pieces then pour the mixture into the cake tin.
Bake in the preheated oven for 30-40 minutes or until the surface is set - it will be cooked when a skewer into the middle comes out with just a little mixture stuck to it.
Remove the brownie from the oven, set aside on a wire rack for about 20 minutes. Cut the brownie into approximately 16 squares.
Meanwhile, for the banana ice cream, remove the frozen bananas from the freezer and leave to defrost slightly for a few minutes. Blend the frozen bananas, vanilla, sugar and half of the buttermilk in a food processor until smooth. While the motor is running, slowly add the remaining buttermilk and continue to blend until smooth and creamy.
For the caramel sauce, place the sugar and butter into a small saucepan and cook until melted and well combined. Pour in the double cream and simmer gently for 3-5 minutes, or until the mixture thickens slightly.
To serve, place a brownie onto serving plates, top with a scoop banana ice cream and pour over the sauce.