Preheat the oven 160C/325F/Gas 3. Grease and line a 450g/1lb loaf tin with non-stick baking parchment.
Use a fork to mash the peeled banana in a mixing bowl. Add the remaining cake ingredients (except the chocolate chips) and beat with an electric hand whisk until combined and smooth.
Stir in the chocolate chips and spoon the mixture into the prepared loaf tin. Level the top.
Bake for 40-45 minutes or until well risen, shrinking away from the sides of the tin and golden-brown.
Set aside to cool for 10 minutes, then remove from the tin and leave to cool on a wire rack.
For the decoration, break the chocolate into small pieces and add to a heatproof bowl.
Melt the chocolate in the bowl over a pan of simmering water (ensuring that the bowl does not touch the water). Stir regularly, until almost completely melted, then remove from the heat. The heat of the bowl will melt any remaining lumps.
Spoon the melted chocolate into a piping bag and pipe zig-zags across the top of the loaf. Set aside for the chocolate to set. Slice and serve.