1. Mix sugar, cornstarch and salt in small heavy saucepan. Gradually whisk in milk and egg yolks until smooth.
2. Cook over medium heat, stirring constantly, until mixture thickens and comes to a boil, about 6 minutes.
3. Reduce heat. Simmer, stirring constantly, 1 minute.
4. Remove from heat. Stir in butter and vanilla.
5. Pour into six 6-oz custard cups.
6. Serve warm or refrigerate until cold. Serve with granola or fruit, if desired.