1. Heat the oven to 170ºC (gas mark 3). Line a swiss roll tin (26cm x 32cm x 1½cm) with greaseproof paper.
2. To make the meringue, combine 55g (2oz) of the caster sugar, the hazelnuts and cornflour in a food processor and process until finely ground.
3. Using an electric mixer, whisk the egg whites and a pinch of salt until they form soft peaks. Keep whisking, gradually adding the remaining caster sugar, until the mixture is thick, white and very glossy.
Carefully fold the nut mixture into the egg whites
until just combined.
4. Spoon the mixture into the tin and smooth the top. Bake for about 25-30 minutes, or until golden and crisp. Leave to cool on the tray for 10 minutes, then turn out onto a wire rack to cool completely.
5. To make the filling, process the hazelnuts and icing sugar in a food processor until a smooth oily paste forms - this will take about 5 minutes and you will need to keep scraping down the sides of the bowl. Add the melted chocolate and process until combined and smooth. Transfer to a bowl, stir in a third of the whipped cream to loosen the mixture, then fold in the remaining cream until just combined.
6. To make the ganache, place the cream and chopped chocolate in a small saucepan and melt over a low heat, stirring occasionally, until smooth. Set aside to cool.
7. To assemble, cut the meringue into equal-sized rectangles, trimming if necessary so all the corners are square. Place one of the pieces on a serving plate and spread with a third of the filling, then repeat with the remaining meringue and filling.
8. Beat the ganache lightly with a wooden spoon then spread over the top and sides of the cake. Put in a cool place to set.