Soak a vanilla pod in the cold milk for 2 hours. Soak the agar-agar in cold water for half an hour. Squeeze water out and pull to pieces. Put it into saucepan with 1 gill milk and 1/2 gill water. Stand on one side of stove and let simmer very gently until quite dissolved. Meanwhile, dissolve chocolate in rest of milk, adding the sugar. Pour the agar-agar into the boiling chocolate through a hot strainer. This is necessary as there is generally a little tough scum on the liquid. If put through a cold strainer, the agar-agar will set as it goes through. When jelly is quite cold, turn out and serve.