Whisk sugar, cornstarch, and salt in a medium saucepan until blended.

Gradually whisk in milk until smooth. Heat over medium, stirring constantly until mixture thickens and bubbles, about 4–6 minutes.

Remove from heat and stir in vanilla. Let cool 2 minutes.

Spoon into serving cups, top with berries, and chill 10–15 minutes or serve warm. Add whipped cream if you want.