Preheat the oven to 350°F (180°C). Roll out the puff pastry on a baking sheet lined with parchment paper, cover with another sheet of parchment, and place another baking sheet on top. Bake for 30 minutes, then let cool.

Heat the milk with the split and scraped vanilla bean. Cover and let steep for 30 minutes.

Whisk the egg yolks with the sugar, add the cornstarch, and mix well. Reheat the milk slightly, then pour it into the egg mixture, stirring well. Pour everything back into the saucepan and cook over medium heat, stirring constantly, until the pastry cream thickens. Transfer the cream to a bowl, cover with plastic wrap directly on the surface, let cool, then refrigerate.

Carefully cut the puff pastry into 3 equal rectangles.

Spread a layer of pastry cream on one rectangle, top with a second rectangle, spread the remaining cream, then place the last rectangle of puff pastry on top. Dust with powdered sugar.

Refrigerate for at least 2 hours before serving.