Preheat the Oven: Start by preheating your oven to 400°F (200°C). This ensures a nice, crisp pastry crust.

Prepare the Pastry: Roll out the puff pastry on a floured surface. Cut the pastry into two equal rectangles and place them in a baking dish lined with parchment paper.

Bake the Pastry: Prick the pastry with a fork (this prevents air bubbles) and bake for 15-20 minutes, or until golden brown. Remove from the oven and let cool.

In a saucepan, combine milk, sugar, and cornstarch. Whisk until fully incorporated. Cook over medium heat, stirring constantly until thickened.

Temper the Egg Yolks: In a bowl, whisk egg yolks. Gradually add the hot milk mixture to the yolks while whisking. Then transfer everything back to the saucepan.

Finish the Custard: Cook the mixture over low heat, stirring constantly, until it thickens once more. Remove from heat and stir in the vanilla extract and butter. Let it cool.

Place one puff pastry layer on a serving platter, spread the custard on top, then gently place the second pastry layer over the custard.

Refrigerate the assembled cake for at least an hour. Before serving, dust with powdered sugar and garnish with fresh fruit if desired.