Grease and flour a loaf tin, otherwise if you’re using a silicone mould there’s no need. Preheat the oven to 180°C/360°F/Mark 4/160°C fan.
In a large mixing bowl, cream the butter and sugar until soft, light and creamy (this is even easier if you beat together in a stand mixer). Gradually add the eggs, one by one until well mixed. Incorporate the flour and baking powder until the batter is smooth.
In another bowl, mash the banana with a fork and transfer half of it to the other bowl. In one of them, add the chocolate powder and chocolate chips and mix well.
Pour the chocolate mix into the bottom of the tin, then pour in the banana batter, then the chocolate again then banana.
Marble the cake by making zig-zags with a fork from one end to the other - or swirl a couple of times in a figure 8 with a skewer. If making this without the teatime glaze, cut the extra banana horizontally (if using) and place on top of the batter. Transfer to the oven and bake for 45-50 minutes. The cake is ready when a knife inserted in the middle comes out clean. If not, bake for another 5 minutes. Leave the cake to cool then remove from the mould to a wire rack to cool.
For the Teatime Glaze (optional):
Melt the chocolate, icing sugar and butter in a bowl placed over a pan of simmering water (bain-marie). When melted, stir in the cream until the glaze is well blended. Leave to cool for about 5 minutes then pour over the cake, evening the glaze with a knife and decorate whatever takes your fancy. I added some mini macaron shells and finally dusted it with gold food powder, just tapping it over with a couple of fingers.
Please resist temptation to eat this straight away, as the marble cake tastes even better the next day. Can keep for 3 days in a cool place stored in an airtight tin or in aluminium foil (although not in the fridge) - if you're lucky not to have tigers around!