Preheat the oven to 150C/300F/Gas 2. Line a 33x28cm/15x11in swiss roll tin with silicone or greaseproof paper.
Place the egg whites into a clean bowl and whisk until soft peaks form when the whisk is removed from the bowl. Gradually add the caster sugar, whisking continuously, until nearly stiff peaks form. Fold in the cornflour and cocoa until well combined.
Spoon the meringue mixture into the prepared swiss roll tin and bake in the oven for one hour, or until cooked but still pliable. Set aside to cool.
In a bowl, mix together the whipped double cream, the tablespoon of icing sugar and vanilla seeds until just combined.
Sieve some icing sugar onto another sheet of greaseproof paper in an even layer, then turn the meringue out on top of the icing sugar.
Spoon the cream onto the meringue and roll up, using the sheet of greaseproof paper to help you lift the meringue. Place the roulade onto a plate and dust with more icing sugar and some cocoa powder.
To serve, thickly slice the roulade and serve with a few fresh raspberries scattered on top.
