To make the custard, pour the milk into a saucepan, preferably non-stick. Using the point of a knife, split the vanilla pod down the length and scrape out the seeds. Cut the pod into three and add to the milk with the seeds. Heat gently, then simmer for 5 minutes.
Put the egg yolks and sugar in a bowl. Whisk together until pale and thick, then gradually whisk in the hot milk. Return to the pan and cook over a gentle heat, stirring all the time, until slightly thickened. Don’t rush this, it should take about 10 minutes, and take care not to let the custard boil or it will curdle. It won’t be as thick as bought custard, it will just lightly coat the back of a wooden spoon.
Strain into a clean bowl, discard the vanilla pods, cover with cling film and cool. (Can be made up to 2 days ahead or frozen.)
To make the meringue, whisk the egg whites until they form stiff peaks, then whisk in a third of the sugar, then another third, and finally the remainder.When the meringue is ready, it will stand in peaks that dip at the end, like a bird’s beak.
Sift the cocoa powder and whisk in. Spoon the meringue into six tea cups, and microwave (500W) individually for 10-15 seconds until firm to the touch. Leave to cool.
Just before serving, spoon the custard into six glasses or glass dishes. Invert each île flotante on to your hand, then slide carefully on top of the custard. Scatter with chocolate curls or grated chocolate and serve. (The desserts can be assembled and chilled for up to 1 hour before serving.)