In a saucepan combine 1/3 cup sugar, cornstarch,and 14 teaspoon salt.
Blend in milk.
Cook and stir till mixture thickens and bubbles-.
Stir a moderate amount of hot mixture into egg yolk; return to saucepan.
Cook and stir till thickened, about 2 minutes more.
Remove from heat; stir in vanilla.
Pour into 1 1/2-quart serving bowl.
Cover surface with plastic wrap; cool.
Chill 2 hours.
Sprinkle remaining sugar over berries.
Place atop flummery.