Preheat the oven to 180°C fan.  Line a 1lb loaf tin with parchment paper.

Make the chocolate sauce by combining the cocoa powder, water and sugar.  Heat gently to combine, whisking out any lumps, then set aside to cool.

For the top of the loaf, select the least mottled banana and set aside with the skin on.

In a large mixing bowl, use an electric whisk to mix together the egg and sugar until thick.  Stir in the vanilla essence.

Peel the rest of the bananas and use a fork to mash the flesh to a pulp.  Add it to the egg and sugar mixture and whisk for a few seconds until fully combined.

In three batches, fold the sieved flour into the liquid mixture, stirring until just combined with no flour pockets remaining.

Peel the reserved banana and slice it in half lengthways.  Roughly chop the other half and add into to the batter.

Spoon two thirds of the batter into the lined tin.  Pour half the chocolate sauce over then cover with the remaining batter.  Place the reserved banana slice cut side facing up on top of the mixture (spoon lemon juice over the cut surface of the banana along with a pinch of brown sugar to give a nice caramelised effect).

Bake in the preheated oven for 50-55 minutes or until risen and golden brown on top. Once baked a skewer inserted into the centre of the cake should come out clean.  Leave the bread in the tin to cool before slicing.  Serve with any remaining chocolate sauce.