Preheat your oven to 400°F (200°C) and roll out two sheets of thawed puff pastry to approximately 9x9 inches on a lightly floured surface. Prick the dough with a fork to prevent rising, then cover each sheet with parchment paper and place another baking tray on top.

Place the prepared puff pastry in the preheated oven and cook for 35-45 minutes, checking for that golden color around the 30-minute mark. The pastry should puff up beautifully and become crisp to the touch when done. Allow to cool completely on a wire rack.

Once cooled, gently remove it from the baking trays. Trim the edges to fit an 8x8 inch pan.

In a medium saucepan, combine the granulated sugar, cornstarch, and kosher salt. Whisk until well mixed, then add the egg yolks, whisking until smooth.

Pour the whole milk into another saucepan and heat over medium-low until steaming but not boiling, about 5 minutes.

Slowly pour the warm milk into the egg mixture while whisking. Return the mixture to the saucepan and cook over medium heat, whisking continuously until thickened, about 5-7 minutes. Add the vanilla and cold cubed butter, stirring until smooth.

Line an 8x8 inch pan with aluminum foil. Place one sheet of baked puff pastry at the bottom, pour the warm custard on top, then place the second layer of pastry on top. Cover with plastic wrap and refrigerate for at least 4 hours or overnight.

After chilling, lift the vanilla slice out of the pan using the foil, trim edges if desired, dust with powdered sugar, and slice into squares.