1. Combine sugar, cornstarch and salt in a medium-size saucepan; stir in boiling water until mixture is well blended.
2. Cook, stirring constantly, until mixture thickens. Add chocolate squares and butter or margarine; continue cooking and stirring until chocolate and butter melt; remove from heat; stir in vanilla.
3. Pour into a medium-size bowl; chill in refrigerator, stirring several times, until thick enough to spread.