Place the egg whites into a spotlessly clean mixing bowl (preferably use a food mixer) and beat on a light speed until beginning to become stiff, then while beating slowly pour the caster sugar, a little at the time . When all the sugar has been incorporated, the cornflour, vinegar and whisk until incorporated. When ready the mixture should be glossy and when the bowl is turned upside down the mixture should remain stationary in the bowl. Stir in the grated chocolate and cocoa powder, just bringing your spoon through the meringue mixture once or twice.
In the meantime, line an oblong baking tray (13 x 9 inches/33 x 23cm) with parchment paper and using a piping bag pipe the mixture out into 4 individual nests.
Bake in the oven for 40-50 minutes until the meringue mixture is very firm to touch but still soft in the middle. I normally leave mine to cool in the oven with the door ajar.
Serve with some lightly whipped cream topped with strawberries.