Preheat the oven to 190℃/375℉. Grease and line a 9x5 inch loaf tin or a 10x4x2.5 inch loaf tin
Melt the dark chocolate in a bowl over simmering water, then allow to cool slightly while you mix the batter.
Mash bananas in a large bowl, then mix in the oil, sugar, vanilla, eggs and sour cream.
In a separate bowl, whisk together the flours, baking soda, baking powder, salt, cinnamon and nutmeg, then fold this into the banana mixture until just combined.
In a small bowl, weigh in 250g of the above mixture and the cocoa powder and combine well.
Get your large bowl with the batter, and scrape the cocoa mix on top, followed by the melted dark chocolate. In 3 swift movements, starting from the perimeter of the bowl and cutting directly through the middle then swooping under and up, politely swirl the 3 components together. Less is more here, as you’ll swirl the batter more when you transfer it to the loaf tin.
Pour the batter into the loaf tin and sprinkle with a good handful of sugar (raw, demerara, turbinado or cane are lovely).
Bake for 10 minutes at 190℃/375℉, then turn the oven down to 180℃/350℉ degrees for 50 mins. After the hour, check to see if its done if a cake tester comes out clean, OR temp the centre of the loaf and if it reads above 200℉ its ready to come out.
