In a medium saucepan, warm whole milk and sugar over medium heat until steaming, but not boiling, about 5 minutes.

In a bowl, whisk cornstarch and egg yolks until smooth and pale yellow, ensuring no lumps remain.

Slowly pour half the hot milk into yolk mixture, whisking constantly for 1 minute to prevent curdling.

Return mixture to saucepan, cook over medium-low heat, stirring constantly until the pudding thickens and coats the spoon, about 3–4 minutes.

Remove from heat, stir in unsalted butter and vanilla extract until silky and glossy.

Divide the warm pudding into individual cups, smoothing tops with the back of a spoon.

Cover and refrigerate for at least 2 hours, or until set and gorgeously creamy.