Prepare the soufflé tin. Dissolve the chocolate in the milk. Melt the butter, add the flour and let it cook for a few minutes without colouring. Add the milk and beat well until smooth. Reheat until the mixture thickens and comes away from the sides of the pan. Allow to cool slightly. Beat in the egg yolks well, one at a time, add the sugar and vanilla essence. Stiffly whisk the egg whites and fold them lightly into the mixture. Turn into the mould; cover, and steam very gently for about 1 hr.