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 to fit an 8×8-inch square pan.

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

Place whole milk in a separate saucepan over medium heat, warming until it begins to steam. Do not boil.

Slowly pour in 1 cup of the warm milk to the sugar and egg mixture, whisking until fully incorporated. Gradually pour the tempered egg yolks into the saucepan with the remaining milk, whisking 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 and stir in vanilla bean seeds and cold, cubed unsalted butter. Whisk until fully mixed in.

Line the 8×8 pan 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 and gently press down.

Cover tightly with plastic wrap or foil and refrigerate for at least 4 hours or overnight.

Lift the slice out of the pan using the foil overhang. Transfer to a cutting board and use a serrated knife to cut into 9 larger or 16 smaller squares. Dust with powdered sugar, then serve.