Bake the pastry layers: Preheat oven to 400°F (200°C). Roll out puff pastry slightly and place on parchment-lined baking sheet. Prick with a fork, cover with parchment, and place another baking tray on top. Bake 15–20 minutes until golden and crisp. Cool, then cut into even rectangles.
Make the pastry cream: In a saucepan, heat milk until almost simmering. In a bowl, whisk egg yolks, sugar, and cornstarch. Slowly whisk in hot milk. Return mixture to saucepan and cook over medium heat, stirring until thick. Remove from heat, stir in butter and vanilla. Cool completely.
Assemble the dessert: Place one puff pastry layer on a serving plate. Spread pastry cream evenly. Repeat layers, finishing with puff pastry on top.
Decorate: Dust with powdered sugar, drizzle chocolate, or top with berries.
Chill and serve: Refrigerate at least 1 hour before slicing with a serrated knife. Serve cold.