In a heavy pan heat the cream with the vanilla pod until scalding hot but not boiling. Remove from heat, cover and leave to infuse for 15 minutes. Remove the pod and scrape seeds into the cream with the tip of a knife. Break up the chocolate and stir into the cream until melted and smooth.
Put the egg yolks and icing sugar into a bowl. Beat with a wooden spoon until well blended and then stir in chocolate cream. Pour into 4 (8-ounce) creme brulee dishes. Stand in a bain-marie. Bake in the oven 30 minutes until firm. Remove and cool. Chill overnight or for up to 48 hours. Sprinkle caster sugar over the top and put under a hot grill to caramelize. Serve within 1 hour.