Preheat the oven to 150C/300F/Gas 2. Line a large baking tray with non-stick baking paper.
In a very clean bowl, whisk the egg whites with an electric whisk on high speed and add 300g/10½oz of the sugar a spoonful at a time, whisking until the mixture is smooth and glossy.
Reduce the speed and add the cornflour and vinegar (thise will give the meringue a chewy inside). Return to a high speed and whisk until the mixture forms stiff peaks when the whisk is removed from the bowl.
Dab a few spots of the meringue directly onto the baking tray and lay the baking parchment on top, using the spots of meringue to stick it down. This stops the meringue being blown about by the oven fan.
Spread the rest of the meringue out in a large circle with a spatula onto the lined baking tray and place in the oven for 10 minutes before turning the oven down to 100C/200F/Gas ¼. Leave the meringue to cook for three hours.
Melt the white chocolate in a heatproof bowl set over a pan of simmering (not boiling) water, making sure the bowl doesn't touch the water.
Whisk the cream in a bowl until soft peaks form when the whisk is removed from the bowl, then whisk in the custard and whisk again until soft peaks form.
Heat the remaining 200g/7oz caster sugar in a saucepan over a medium heat until liquid and golden-brown. Set the caramel aside to cool.
Top the meringue with the melted white chocolate and allow to cool slightly before spreading the cream over the top.
Arrange the raspberries on top and then drizzle the caramel over.