Line a loaf tin with baking paper. Dissolve the cocoa powder in 4 tablespoons of boiling water and set aside to cool. Preheat the oven to 180°C / 160°C (fan oven) / 360°F / Gas Mark 4.

In a small bowl, mix the bananas, eggs and vanilla. Sift the flour into a separate bowl and rub in the spread with your fingers. Stir in the sugar and the banana mixture. Put a third of the mixture in the small bowl and stir in the cocoa mixture.

Put random dollops of each cake mixture in the tin until youve used all the mixture, then swirl with a knife to create a marbled effect.

Bake for 55 minutes or until golden and a skewer inserted into the centre of the cake comes out clean. Leave for 10 minutes, then turn out onto a wire rack to cool.