In a saucepan, whisk together milk, cornstarch, and sugar until smooth.
Place the pan over medium heat and stir constantly until the mixture thickens.
Remove from heat and stir in the butter and vanilla extract until fully combined.
Let the mixture cool for 10–15 minutes, then gently fold in the whipped cream.
Spoon the dessert into serving cups and refrigerate for at least 2 hours before serving.