Preheat your oven to 400°F (200°C). Line two baking trays with parchment paper. Roll out the puff pastry sheets slightly to about 9×9. Prick all over with a fork to prevent puffing during baking. Place the pastry sheets on the trays, then cover with another sheet of parchment and a second tray to weigh them down.

Bake for 35-45 minutes, or until golden and crisp. Set aside and cool completely.

Using a serrated knife, carefully trim the edges of the puff pastry to fit an 8×8-inch square pan.

In a medium saucepan, whisk together the granulated sugar, cornstarch, and kosher salt. Add the egg yolks, then whisk until thoroughly combined.

Place the whole milk in a separate saucepan over medium heat, then warm until it begins to steam.

Slowly pour in 1 cup of the warm milk to the sugar and egg mixture, whisking continuously until fully incorporated. Gradually pour the tempered egg yolks into the saucepan with the remaining milk, then whisk until fully mixed.

Continue cooking over medium heat, whisking constantly, until the mixture thickens and begins to bubble, about 5-7 minutes. Remove from heat, then stir in the vanilla bean seeds and cold, cubed butter. Continue whisking until the butter is completely mixed in.

Line the 8×8 pan fully on the bottom and all sides with aluminum foil. Place one piece of cooked puff pastry on the bottom. Pour the warm custard on top, then smooth into a flat, even layer. Place the second piece of puff pastry on top, then gently press down to adhere. Cover tightly with plastic wrap or foil, then refrigerate for at least 4 hours or overnight.

Gently lift the slice out of the pan using the overhang of foil. Transfer to a cutting board and carefully cut into squares. Dust with powdered sugar, then serve.