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.