Combine sugar, cornstarch, and salt in a small saucepan; gradually stir in milk. Cook over medium heat, stirring constantly, until mixture comes to a boil. Cook 1 additional minute, stirring constantly. Remove from heat.
Beat egg yolk until thick and lemon colored. Gradually stir about 1/4 of hot mixture into yolk; add to remaining hot mixture, stirring constantly. Bring mixture to a boil over medium heat and cook 1 minute, stirring constantly. Remove from heat; stir in butter and vanilla. Pour mixture into two 6-ounce custard cups. Chill. Yield: 2 servings.