MIX sugar, cornstarch and salt in small heavy saucepan. Gradually WHISK IN milk and egg yolks until smooth.

COOK over medium heat, stirring constantly, until mixture thickens and comes to a boil, about 6 minutes. REDUCE heat; SIMMER, stirring constantly, 1 minute. Remove from heat. STIR IN butter and vanilla.

POUR into six 6-oz. custard cups. SERVE warm or refrigerate until cold. SERVE with granola or fruit, if desired.