Cut the puff pastry into rectangles—you'll need 3 per person. Prick them with a fork, place them on a baking rack, cover with another sheet of parchment paper and a second rack, then bake for about 20 minutes at 350°F (180°C). Let cool.

Make the pastry cream: Heat the milk with the vanilla bean, then cover and let steep for 10 minutes. Meanwhile, whisk the egg yolks with the granulated sugar, then add the cornstarch. Slowly pour in the hot milk while mixing. Pour the mixture back into the saucepan and cook over low heat, whisking constantly, until thickened. Transfer the cream to a bowl and let cool. Place the cream in a piping bag.

Take the first rectangle of puff pastry and pipe rosettes of pastry cream on top. Rinse and dice the strawberries (save a few for decoration). Sprinkle the diced strawberries over the cream. Repeat with the second rectangle of puff pastry.

Dust the third rectangle of pastry with powdered sugar, then place it on top of the mille-feuille.

Decorate with the reserved strawberries and a few mint leaves for a touch of green. Chill until ready to serve.