In medium saucepan over medium heat, heat milk until bubbles form at edges. In a bowl, combine sugar, cornstarch and salt. Pour into hot milk, a little at a time, stirring to dissolve. Continue to cook and stir until mixture thickens enough to coat the back of a metal spoon. Do not boil. Remove from heat, stir in vanilla and butter. Pour into serving dishes. Chill before serving.
