Mix the butter with the powdered sugar into a fluffy cream. Peel the apples and mix them with the cinnamon. Add the butter cream, as well as the finely ground biscuits and walnuts, then stir well.
Transfer the mixture to a form, even it out well on top, sprinkle with hazelnuts and cover with a glaze from the beaten cream, mixed with the melted and cooled chocolate. Keep the cake in the fridge until it's time to serve.