1. Prepare the Puff Pastry Layers

1. Preheat the Oven: Preheat your oven to 400°F (200°C). Line two baking sheets with parchment paper.
2. Roll Out the Pastry: Lightly flour a clean surface and roll out each sheet of puff pastry until it’s approximately 10×10 inches. Prick the surface all over with a fork to prevent excessive puffing during baking.
3. Cut and Bake: Cut each sheet into equal rectangles, approximately 3×4 inches, yielding 18 pieces total. Place the rectangles on the prepared baking sheets, leaving space between them. Brush with beaten egg for a golden finish, if desired.
4. Bake: Bake for 15-20 minutes, or until golden brown and crisp. Rotate the trays halfway through for even baking. Remove from the oven and allow the pastry to cool completely.

2. Make the Vanilla Custard

1. Heat the Milk: In a medium saucepan, heat the milk over medium heat until it’s just about to simmer (small bubbles will form around the edges). Do not boil.
2. Mix the Dry Ingredients: In a separate bowl, whisk together the sugar and cornstarch.
3. Temper the Egg Yolks: In another bowl, whisk the egg yolks until smooth. Gradually add the sugar and cornstarch mixture, whisking to combine. Slowly pour in a small amount of the hot milk (about ½ cup) into the yolk mixture, whisking constantly to temper the eggs.
4. Cook the Custard: Pour the tempered yolk mixture back into the saucepan with the remaining milk. Cook over medium heat, whisking constantly, until the mixture thickens to a pudding-like consistency (about 3-5 minutes).
5. Add Butter and Vanilla: Remove the saucepan from the heat and stir in the butter and vanilla extract until smooth. Transfer the custard to a bowl, cover with plastic wrap (press it directly onto the surface to prevent a skin from forming), and chill for at least 2 hours.

3. Assemble the Vanilla Custard Cream Squares

1. Layer the Pastry and Custard: Place one piece of puff pastry on a serving plate or tray. Spread a generous layer of chilled custard over the pastry, then top with another piece of pastry. Repeat the layering process, ending with a puff pastry piece on top.
2. Decorate: Dust the top layer generously with powdered sugar. For added flair, drizzle with melted chocolate or caramel sauce, or garnish with fresh fruit.

4. Chill and Serve

Refrigerate the assembled dessert for at least 1 hour before serving. This helps the custard set and makes slicing easier. To serve, use a sharp knife to cut the stack into squares, wiping the blade clean between cuts for neat edges.