Not only is this one of the most scrumptious things to eat, it’s also a total pleasure to make. The amount of smiles and oohs and aahs you're going to get when you serve it make it simply the best. If you can, try and use slightly overripe bananas as they’ll give you a treacly, caramelly intensity that works so well. The chocolate butter is painfully enjoyable, but my nutritionist, Laura, has requested that I suggest just having a smidgen with a slice of the loaf, so I'm sure you'll all respect her wishes when tucking into this delight.
Preheat the oven to 170°C/325°F/gas 3. Spread the walnuts out on a baking sheet and pop them in the oven to gently toast for around 5 minutes, or until they smell fantastic. Meanwhile, quickly mash up the bananas with a fork or potato masher, so you’ve got a mixture of smooth and chunky, and put them aside.
Cream the butter and sugar together in a food processor, or by hand, until smooth and pale, and beat in the eggs one by one, scraping the sides as you go so everything gets mixed together, then spoon into a large bowl. Take the toasted walnuts out of the oven and put them on a chopping board. Quickly run a knife through them, leaving some halved and others fairly fine so you get a good range of textures. Add the mashed bananas and chopped up walnuts to the bowl of batter, then sift in the flour, baking powder, bicarbonate of soda, cinnamon and salt. Mix everything together until you have a nice smooth batter.
Tear off a metre of greaseproof paper, scrunch it and wet under a tap. Drizzle a little olive oil over both sides and rub that in, then push the scrunched greaseproof into a 1-litre loaf tin, getting it right down into the corners (it’s ok if it’s a bit scruffy). Transfer the batter to the tin and cook on the middle shelf in the oven for an hour. After this time, test the loaf by poking a skewer into the middle. If the skewer comes out clean the loaf is cooked, if not put the tin back in the oven for another few minutes.
While the loaf bakes, make your chocolate butter. Bash the chocolate up, pop it in a heatproof bowl and melt it slowly over a pan of gently simmering hot water. Once melted, carefully take the bowl off the pan, stir in the orange zest and put to one side to let the chocolate cool down a little.
Cream the butter and icing sugar together, then beat in the cooled chocolate until blended. Once you’ve got a nice even mixture, transfer it to a little pot or a cute butter dish and sprinkle on a tiny pinch of sea salt from a height to give it a little salty kick. Either put in the fridge to set for a few hours, or serve right away smeared on a slice of warm banana loaf. Keep any leftover chocolate butter in a jar in the fridge for another day, and just take it out of the fridge to soften before serving.