Make the pastry. Add the flour, cocoa, sugar and butter to a large bowl, then rub in with your fingertips or an electric mixer until you have fine crumbs. Mix in enough water to form a soft but not sticky dough. Roll out on a floured surface to fit a fluted loose-bottomed tart tin, 20 cm (8 inches) in diameter, 5 cm (2 inches) deep. Line the tin (see
Meanwhile, whisk the egg yolks and 50 g (2 oz) of the sugar, the cocoa powder and cornflour together in a large bowl. Pour the milk into a saucepan and bring to the boil. Gradually whisk the hot milk into the egg yolk mixture and then pour it back into the saucepan, whisking constantly until thick. Stir in the vanilla, then pour into the tart case.
Whisk the egg whites until stiff, then gradually whisk in the remaining sugar until it has all been added and the meringue is very thick, then fold in the grated chocolate. Spoon the meringue over the pie, shape into swirls, then bake at 180°C (350°F), Gas Mark 4, for 15 minutes until the meringue is golden. Cool before serving sprinkled with extra grated chocolate to decorate.