For the puff pastry layers: Preheat your oven to 400°F (200°C). Roll out the puff pastry sheets on a lightly floured surface to smooth out any creases. Trim the edges and cut into an even rectangle or square. Place on a baking sheet, brush with a beaten egg, and sprinkle with sugar. Bake for 15-20 minutes until golden brown and puffed up. Remove and let cool.

1. For the vanilla custard filling: Heat the milk in a medium saucepan over medium heat until warm but not boiling. In a bowl, whisk egg yolks, sugar, and cornstarch until smooth. Gradually add warm milk to egg mixture, whisking constantly, then pour the egg mixture back into the saucepan. Cook over medium heat until thickened. Remove from heat, stir in vanilla extract and butter. Allow to cool to room temperature and refrigerate for 1-2 hours.
2. Assembling the cake: Place one layer of puff pastry on a platter. Spread chilled vanilla custard on top. Place the second puff pastry sheet on top and press down lightly. Dust with powdered sugar or cocoa powder if desired. Slice and serve.