Preheat the oven according to the puff pastry package instructions.

Roll out the thawed puff pastry sheets and fit them into a baking dish, pressing down to adhere.

In a saucepan, combine granulated sugar, cornstarch, and kosher salt.

Gradually whisk in warm whole milk until smooth and heated through.

In a separate bowl, whisk the egg yolks.

Slowly add a small amount of the warm milk mixture to the egg yolks, whisking continuously to temper them.

Return the tempered egg mixture to the saucepan and cook over medium heat, stirring constantly until the custard thickens.

Stir in the scraped vanilla bean seeds (or vanilla bean paste) and cold cubed butter until fully incorporated.

Pour the custard into the prepared puff pastry and smooth the top.

Bake according to the puff pastry package instructions or until golden brown.

Allow to cool, then dust with powdered sugar before serving.