Melt the chocolate in a bowl over hot water.
Beat the egg whites until very stiff. Add the caster sugar and whisk again until stiff and the sugar is dissolved. Bring the milk to a simmering point in a wide flat pan. Shape the egg white mass into a round flat cake and poach this gently in the simmering milk for a few minutes, turn it carefully with a fish slicer or long spatula and poach the other side. Leave to drain on kitchen paper or a clean cloth.
Beat the egg yolks and half the sugar into the melted chocolate and pour the slightly cooled milk over the mixture. Place over hot (not boiling) water in a double boiler. Cook stirring constantly until the mixture thickens to the consistency of thin cream. Pour into a glass bowl and place the egg whites on top. Chill.
Ten minutes before serving, caramelize the remaining sugar in a heavy saucepan and pour over the egg whites.