Whisk the egg whites in a large clean bowl until stiff. Whisk in the sugar, a tablespoonful at time, until it has all been added. Whisk again until the meringue mixture is thick and glossy. Fold in the cornflour and ground hazelnuts then spoon the mixture into a large piping bag fitted with a 1 cm (½ inch) plain piping tube.
Draw a 23 cm (9 inch) circle on 3 sheets of nonstick baking paper. Starting in the centre of each prepared circle, pipe the mixture in a continuous coil, finishing just within the outer line. Bake all 3 in a preheated oven, 150°C (300°F), Gas Mark 2, for 1–1¼ hours until lightly golden and dried out. Remove from the oven and transfer to a wire rack to cool completely.
Heat the chocolate and cream in a heatproof bowl set over a saucepan of simmering water, stirring until the chocolate has melted. Remove from the heat and leave to cool then chill for 1 hour until thickened.
Make the chocolate hazelnuts. Using a fork, dip the hazelnuts into the melted chocolate until coated. Leave to set on baking paper.
Beat the chocolate filling until light and fluffy and use it to sandwich the 3 meringue layers together. Decorate with the chocolate hazelnuts and serve dusted with cocoa powder.