1. Combine sugar, cornstarch and salt in a heavy, medium-size saucepan; stir in boiling water until well blended.
2. Cook, stirring constantly, until mixture thickens and bubbles for 1 minute. Add the chocolate squares and butter or margarine; continue cooking and stirring until chocolate and butter or margarine melt; remove from heat; stir in vanilla.
3. Pour into a medium-size bowl; chill, stirring several times, until spreadable.