Preheat the oven to 200C/180C Fan/Gas 6. Line a 2lb loaf tin.
In a large bowl, mash the bananas. Then add the butter, both sugars and the egg (ensure the butter has cooled a little so the egg doesn’t cook). Mix with a wooden spoon until combined.
Add the dry ingredients: flour, baking powder, baking soda and white chocolate. Stir until just combined.
Scoop out about a cup of batter into another bowl. Add the melted dark chocolate and cocoa powder. Stir until combined.
Use tablespoons to scoop the batter into the lined tin, alternating between each time until all used up. Use a spoon to lightly swirl the mixtures together (not too much so that it actually combines).
Bake for 40-45 minutes, until a skewer inserted comes out clean. Let cool on a wire rack before slicing and serving.