Spread the hazelnuts out on a baking sheet and toast in a preheated oven, 160°C (325°F), Gas Mark 3, for 5–10 minutes until golden. Leave to cool completely, then grind very finely.
Put the chocolate in a heatproof bowl set over a saucepan of hot water and leave to melt. Whip the cream in a bowl until it holds its shape, then fold in the nuts. Whisk the egg yolks in a large bowl with 2 tablespoons of the icing sugar until pale and creamy. Whisk the egg whites in a separate bowl until soft peaks form, then add the remaining icing sugar, spoonful by spoonful, whisking well after each addition, until the mixture is very thick.
Stir the melted chocolate into the egg yolk mixture. Fold in the cream mixture, then the meringue mixture. Turn into 6 x 175 ml (6 fl oz) moulds and freeze for 6 hours until firm.
Transfer to the refrigerator for 10 minutes before serving to soften slightly. Decorate with chocolate curls and dust with cocoa powder. Serve with dessert biscuits.