Heat 100g of the sugar in a frying pan over medium–low heat, until it has melted. Add the hazelnuts, stirring to coat the nuts with the caramelised sugar. Continue cooking for 5 minutes, shaking the pan until the hazelnuts are completely coated with the caramelised sugar. Pour the hazelnuts back onto the baking tray and allow to cool completely. Crush them coarsely in a food processor and set aside 3 tbsp for decorating the top of the cake. Continue to process the remaining hazelnuts until they are finely ground.