Preheat the grill to medium, then toast the hazelnuts on a baking tray until golden, taking care not to burn them. Transfer to a dish and leave to cool.
Roughly chop a few hazelnuts and set them aside for decoration. Use a clean tea towel to rub the bitter skins off of the toasted nuts. Finely chop or grind the hazelnuts in a food processor or blender with a tablespoon or two of the caster sugar.
Preheat the oven to 140 degrees C. Line 2 baking trays with baking paper and draw a 20 cm circle on each piece of paper.
Place the egg whites in a large bowl and beat with an electric mixer until they form medium-stiff peaks. Gradually add the caster sugar a tablespoon at a time, then continue to beat for 1–2 minutes or until the meringue is very thick and glossy. Fold in the hazelnuts with a large metal spoon or whisk.
Divide the meringue mixture between the baking trays and spread evenly within the drawn circles. Bake for 40 minutes, then turn the baking trays around in the oven so that the meringues colour evenly. Bake for another 35 minutes or until the meringues are set and can be removed easily from the paper. Loosen them from the paper, then leave to cool completely on the baking trays.
While the meringues are baking, make the pastry cream. Pour 1¼ cups milk into a medium saucepan and bring to the boil. Remove the pan from the heat and add the vanilla pod, scraping out the seeds with the tip of a paring knife and adding them to the pan. Let the mixture infuse for 30 minutes.
Place the egg yolks and sugar in a medium bowl and beat to combine. Sift in the flour and cornflour and whisk to mix. Bring the milk just to the boil, then gradually pour it into the egg yolk mixture in a steady stream, whisking constantly.
Pour the mixture back into the saucepan and bring to the boil, stirring constantly with a wooden spoon. When thick, simmer gently for 1 minute, still stirring constantly. Remove from the heat. Remove the vanilla bean pod.
Cover the surface of the pastry cream with wet greaseproof paper and leave to cool. (Both the meringues and pastry cream can be made a day in advance; store the meringues in an airtight container and the pastry cream in a covered bowl in the refrigerator.)
Assemble the cake no more than 1 hour before serving. Place one of the meringues on a serving plate, flat side facing upwards, and spread with the pastry cream. If the cream is too thick to spread, beat in the extra milk. Sprinkle with two-thirds of the raspberries, then top with the second meringue. Decorate with the remaining raspberries, the reserved toasted hazelnuts and a light dusting of icing sugar. Cut into thick slices with a sharp knife to serve.