For the filling, put the egg yolk, sugar, cornflour and flour into a small bowl; beat well. Stir in 4 teaspoons milk until smooth. Heat the remaining milk until hot but not boiling. Stir into the filling mixture. Pour mixture into a saucepan; cook over a medium heat, stirring constantly, until a thick, smooth custard forms. Add chocolate; stir until melted. Pour into a bowl and cover surface with plastic wrap. Leave to cool completely. Whisk egg white until stiff. Fold into the custard.