Preheat the oven to 180*C. Grease and line a deep rectangular brownie tray with greaseproof paper in two strips to make it really easy to remove the baked banana cake.
In the bowl of a stand mixer, or large bowl cream together the butter and sugar until light and fluffy. This should take 6-10 minutes.
Meanwhile, remove both black ends of the banana, and roughly the banana and peel. Add to a high speed blender with the buttermilk and vanilla bean paste and blend until smooth. Don’t worry if it turns a bit beige – the banana peel is very prone to oxidization, but nothing’s gone wrong.
Now that the butter and sugar are light and fluffy, add one egg at a time, beating well after each addition.
Add the salt, and then on a low speed, alternate additions of the flour and banana mixture in about 2-3 stages. Fold in the chocolate chunks and add to the lined baking tray.
Bake for 30 minutes until a skewer comes out clean (be careful not to confuse batter with melted chocolate!). Leave to cool for 10 minutes in the tray before transferring to a wire rack.