Combine sugar, cornstarch, and salt. Stir in milk. Cook and stir until thickened and bubbly. Stir a small amount into the egg yolk; return all to saucepan. Cook and stir for 2 minutes or until thickened. Remove from heat; stir in vanilla. Pour into a serving bowl. Cover the surface with plastic wrap and cool; then chill. Remove plastic and garnish with berries before serving.