Prepare and bake the puff pastry

• Preheat the oven to 180 °C.
• Roll out the puff pastry on a floured surface and cut it into about 12 rectangles.
• Prick the rectangles with a fork and place them on a parchment-lined baking tray.
• Cover with another piece of parchment and weight down with a second baking tray.

Bake until golden and crisp

• Bake the puff pastry for 20 minutes.
• Remove the top baking tray and parchment, then bake for an additional 5-10 minutes until golden and crisp.

Make the pastry cream

• Heat the milk with the vanilla bean or extract in a saucepan, then discard the vanilla pod if used.
• Whisk together the egg yolks, sugar and cornstarch in a bowl, then gradually whisk in the hot milk.
• Return to the saucepan and cook over medium heat, stirring, until the mixture thickens and bubbles.
• Stir in the butter, then cool with plastic wrap pressed against the surface.

Assemble the mille-feuille

• Transfer the cooled cream to a piping bag.
• Pipe the cream onto one puff pastry rectangle.
• Place another rectangle on top and repeat with more cream, finishing with the final puff pastry rectangle.

Finish and serve

• Dust with powdered sugar before serving.