Break the digestives into the bowl of a processor, add the butter and a 15ml tablespoon of Nutella, and blitz until it starts to clump. Add 25g/3 tablespoons of the hazelnuts and continue to pulse until you have a damp, sandy mixture.Tip into a 23cm/9inch round springform and press into the base either using your hands or the back of a spoon. Place in the fridge to chill.Beat the cream cheese and icing sugar until smooth and then add the remaining Nutella to the cream cheese mixture, and continue beating until combined.Take the springform out of the fridge and carefully smooth the Nutella mixture over the base. Scatter the remaining chopped hazelnuts on top to cover and place the tin in the fridge for at least four hours or overnight. Serve straight from the fridge for best results.