Prepare the Pastry Layers:

1. Preheat the Oven: Preheat your oven to 375°F (190°C). Line two baking sheets with parchment paper.
2. Bake the Puff Pastry: Place the puff pastry sheets on the baking sheets and prick them all over with a fork to prevent excessive puffing.
3. Bake: Bake for 15-20 minutes, or until golden brown and crisp. Let them cool completely.
4. Trim to Fit: Once cooled, trim one of the sheets to fit an 8×8-inch baking dish. Set the other sheet aside for the top layer.

Make the Custard Filling:

5. Heat the Milk and Cream: In a saucepan, heat the milk, heavy cream, and half of the sugar over medium heat until warm (do not boil).
6. Mix Egg Yolks and Cornstarch: In a separate bowl, whisk the egg yolks, remaining sugar, cornstarch, and salt until smooth.
7. Temper the Eggs: Gradually whisk a ladle of the warm milk mixture into the egg mixture to temper it. Slowly pour the tempered mixture back into the saucepan while whisking continuously.
8. Cook Until Thick: Continue cooking over medium heat, stirring constantly, until the custard thickens (about 5-7 minutes). Remove from heat.
9. Add Vanilla and Butter: Stir in the vanilla extract and butter until smooth and fully incorporated.
10. Cool Slightly: Let the custard cool for about 10 minutes, stirring occasionally to prevent a skin from forming.

Assemble the Squares:

11. Layer the Pastry and Custard: Place the trimmed pastry sheet into an 8×8-inch dish. Pour the warm custard over the pastry, spreading it evenly.
12. Add the Top Layer: Carefully place the second puff pastry sheet on top, pressing gently to set.
13. Chill: Refrigerate for at least 1 hour, or until fully set.

Serve:

14. Dust with Powdered Sugar: Before serving, generously dust the top layer with powdered sugar for a beautiful finish.
15. Slice into Squares: Use a sharp knife to cut into 12 even squares.