Creating Vanilla Custard Puff Pastry Cake can be a straightforward process if you follow these simple steps:

1. Preheat the Oven: Start by preheating your oven to 400°F (200°C).
2. Prepare Puff Pastry: Roll out the thawed puff pastry sheets on a lightly floured surface. Cut them into two rectangles of equal size, roughly 9×13 inches each.
3. Egg Wash: Place one pastry rectangle on a baking sheet lined with parchment paper. Brush it with egg wash for a golden finish.
4. Bake Pastry: Place the baking sheet in the oven and bake for 20-25 minutes or until the pastry is puffed and golden. Remove from the oven and let it cool completely on the baking sheet.
5. Make Custard: In a medium saucepan, whisk together milk, sugar, and cornstarch over medium heat. Cook until the mixture starts to thicken.
6. Add Egg Yolks: In a separate bowl, whisk the egg yolks. Gradually add a small amount of the hot milk mixture to the yolks, whisking continuously to temper them. Then return this mixture to the saucepan.
7. Cook Custard: Continue cooking on low heat until the custard is thickened and bubbles form. Remove from heat and stir in vanilla extract and butter.
8. Cool Custard: Transfer the custard into a clean bowl, cover with plastic wrap touching the surface, and refrigerate until completely cooled.
9. Assemble Cake: Once everything is cool, transfer one layer of puff pastry to your serving plate. Spread the cooled custard over the puff pastry evenly.
10. Add Second Layer: Place the second layer of puff pastry on top of the custard. You can also brush this with egg wash, if desired.
11. Dust with Powdered Sugar: Just before serving, dust the top layer with powdered sugar.

The Vanilla Custard Puff Pastry Cake is now ready for serving.