Base (Crust): Melt the butter and chocolate together then stir it into the chocolate biscuit crumbs. Presss into the base of a buttered cake tin.
Middle (Mousse): Combine the melted chocolate and yolks in a medium bowl. Lightly whip the cream then fold into the chocolate mixture.
Whisk the egg whites until stiff. Gently fold the egg whites into the chocolate cream mixture. Pour the mixture into the pastry case then refrigerate for 3 hours or until set.
Top (Mock Cream): Blend together the cream, sugar, eggs and cornflour in a medium saucepan until smooth. Stir over low heat until just starting to thicken. Do not allow to boil.
Remove from the heat, stir in the butter, vanilla and chocolate until smooth. Cool. Refrigerate until cold.
To Serve: Whisk the cold custard until really thick and swirl over the set mousse. Decorate with the chocolate curls and dust with cocoa, if desired.