1. Break up the chocolate. Add half the water and dissolve over gentle heat.
2. Mix the cornflour and salt to a smooth cream with little of the remaining cold water.
3. Heat the rest; when it starts boiling, pour over the blended cornflour, stirring well.
4. Add the dissolved chocolate, butter and sugar and cook for 4 to 5 minutes stirring and beating well. Lastly, add a few drops of vanilla essence.