Vanilla and Lemon Pastry Cream:
In a saucepan, bring the milk to a boil with the split vanilla bean and lemon zest. Let it steep off the heat for 10 minutes.
In a bowl, whisk the egg yolks with the sugar until the mixture pales. Add the flour and mix well.
Remove the vanilla bean from the milk, then pour the hot milk over the egg mixture, whisking constantly.
Return everything to the saucepan and cook over medium heat, stirring constantly, until thickened.
Stir in the lemon juice and butter, then mix well until the cream is smooth.
Let the pastry cream cool, then refrigerate until completely cold.
Assembly:
Cut the puff pastry sheets into equal rectangles.
Spread a generous layer of pastry cream over one rectangle of puff pastry. Repeat with a second rectangle of pastry and another layer of cream.
Cover with a third rectangle of puff pastry.
For the icing, mix the icing sugar with the lemon juice until smooth and thick. Spread the icing over the top of the mille-feuille.
With the melted dark chocolate, draw parallel lines on the icing, then use a toothpick to draw perpendicular lines to create a marbled pattern.
Let rest in the refrigerator for at least 1 hour before cutting and serving.
