Preheat the oven to 180°c. Place 12 paper cases in a 12 hole cake tin.
Put the sugar and oil in a mixing bowl and whisk together. Add the eggs and whisk again until smooth.
Add the banana, flour and cocoa powder and whisk until just mixed.
Spoon the mixture into the paper cases and bake for 10-12mins until the cakes are well risen and the tops spring back when pressed in the middle. Leave to cool in the tray.
In the meantime, put the white chocolate in a heatproof bowl and set over a pan of simmering water, stirring regularly until it has melted.
Transfer the cakes to a plate and then drizzle the chocolate all over the top with a teaspoon.