Grease a 10cm x 20cm (base measurement) loaf pan and line the base and sides with baking paper, allowing the 2 long sides to overhang.
Combine the flour and sugar in a large bowl. Add the buttermilk, butter, eggs and banana. Stir to combine. Stir in the date and walnut. Spoon the mixture into the prepared pan and smooth the surface.
Place an upturned heatproof saucer or ramekin in a slow cooker. Place loaf pan on the saucer or ramekin. Bake for 3 hours on high (or 5 hours on low) or until a skewer inserted in the centre comes out clean. Set aside in the pan for 10 mins before turning onto a wire rack to cool.
Cut the loaf into slices. Serve with caramelised banana and extra butter.