Preheat the oven to 180C/fan 160C/350F/Gas 4 and lightly grease a 20x33cm/8x13in square-cornered Swiss roll tin.
Place all the ingredients for the shortbread in a food processor and pulse to form a smooth dough.
Press the mixture firmly into the base of the tin, to create a flat surface and clean edge. Prick all over with a fork and bake for 20-30 minutes, or until lightly golden-brown and firm.
For the topping, grind the hard banana chips in a food processor to a coarse powder. Set aside.
To make the caramel, place the butter, sugar, syrup and condensed milk into a saucepan and stir over a low heat until the butter has melted, stirring all the time. Boil the mixture gently for 10 minutes, stirring all the time until deep golden-brown and thickened, stirring in the mashed banana a few minutes before the end.
Remove from the heat and allow to cool slightly before stirring through the banana chips and reserved banana powder. Pour over the baked shortbread base and sprinkle over the sea salt flakes. Allow to cool slightly while preparing the chocolate topping.
Melt the chocolate in a bowl set over a pan of simmering water (do not allow the bottom of the bowl to touch the water). Pour the chocolate over the banana salted caramel layer and leave to set in the fridge.
Cut into bars with a very sharp, heavy knife and stack up on a serving plate. Decorate with gold leaf-covered hard and chewy banana chips if you like.