For the pastry: First melt the chocolate in a bowl over a saucepan of hot but not boiling water. Rub the margarine into the flour, bind to a dough with the chocolate and the tablespoon of water.
Carefully press into a 23cm (9 in) fluted flan tin, ensuring that there are no thick corners in the base. Bake in a 180 C / Gas 4 oven for 20-25 minutes. Leave to cool, then carefully remove the pastry case from the tin.
For the custard: In a saucepan, blend together the cream, sugar, eggs and cornflour until smooth. Stir continuously over a very low heat until just starting to thicken. Do not allow to boil.
Remove from the heat, stir in the margarine, vanilla and chocolate, then leave to cool. Place in the fridge and leave until very cold.
For the mousse: Using a vegetable peeler make 50g of chocolate curls and keep to the side. Melt the remaining chocolate in a bowl suspended over a saucepan of simmering water, then stir in the egg yolks. Remove from heat. Fold some of the whipped cream into the chocolate then fold this mixture into the remaining cream.
Dissolve the gelatine in 2 1/2 tablespoons hot water. Whisk the egg whites until stiff. Fold the dissolved gelatine into the chocolate cream mixture with the egg whites. Pour the mixture into the pastry case and leave to set.
Whisk the cold custard until really thick and swirl over the set mousse. Decorate with the chocolate curls and if liked, a little cocoa sieved over the top.