Preheat oven to 180°C Spread the hazelnuts over an oven tray. Roast in preheated oven for 10 minutes or until lightly toasted. Place hazelnuts in a clean tea towel and rub to remove the skins. Coarsely chop.
Line the base of a 20cm round (base measurement) springform pan with baking paper. Rub the butter over the base and side of the pan. Sprinkle with the chopped hazelnuts.
Place dates and bicarbonate of soda in a heatproof bowl. Pour over the boiling water and set aside for 5 minutes to soak. Transfer to the bowl of a food processor. Add the banana, sugar, extra butter and flour and process until almost smooth. Pour into the prepared pan.
Bake in preheated oven for 1 hour or until a skewer inserted in the centre comes out clean. Remove from oven and set aside for 10 minutes to cool slightly.
Meanwhile, to make the butterscotch sauce, combine the sugar, cream and butter in a medium saucepan over medium heat. Cook, stirring, for 5 minutes or until butter melts and mixture is well combined and heated through.
Pour half the sauce over the pudding. Cut into wedges, drizzle with remaining sauce. Serve immediately with cream or ice-cream, if desired.