1. Preheat the oven to 180°C (350°F). Line a 23cm loaf tin* and grease lightly. Mash the bananas and set aside. Chop the chocolate and set it aside.

2. Add the sugar and melted butter to a large bowl and mix on high speed until creamy. Add the egg and egg yolk and mix until well combined. Mix the flour with cocoa powder, baking soda, and salt. Add it together with the buttermilk and vanilla extract to the bowl and mix until just combined. Add the mashed bananas and chopped chocolate and mix in. Pour the batter into the prepared loaf pan, smooth out the top.

3. Bake the banana bread for about 55-60 minutes or until a wooden skewer inserted into the center comes out clean. Let it cool down in the pan for some time, then remove and let it cool down completely on a wire rack.