1) In a saucepan, mix together sugar, cornstarch, and salt
2) Stream in boiling water and stir until well blended.
3) Heat the sugar mixture stirring constantly until mixture thickens.
4) Add chocolate squares and butter or margarine; continue cooking and stirring until chocolate and butter melts
5) Take the pan away from heat and stir in vanilla.
6) Transfer the chocolate mixture into a medium-size bowl.
7) Refrigerate, stirring several times, until thick enough to spread.