Prepare Pastry Dough:

If making homemade rough puff pastry, complete through the second refrigeration. If using store-bought frozen puff pastry, thaw as per package instructions. Keep dough refrigerated until ready to use.

Make the Pastry Cream:

In a heatproof bowl, whisk together egg yolks and cornstarch until thickened and combined.

Combine milk and sugar in a saucepan over medium heat. Whisk until the sugar dissolves, then bring to a gentle simmer. Remove from heat.

Slowly pour the warm milk mixture into the egg mixture, whisking constantly to prevent scrambling.

Strain the mixture through a sieve into the saucepan and return it to medium heat. Whisk constantly until the mixture thickens into a custard consistency (about 1–2 minutes).

Remove from heat, then whisk in butter, vanilla extract, vanilla bean seeds, and salt.

Cool for 10 minutes, then transfer to a bowl. Cover with plastic wrap or parchment directly on the surface to prevent a skin from forming. Refrigerate for at least 3 hours, up to 24 hours.

Preheat the Oven:

Preheat to 400°F (204°C) and line a 12×17-inch baking sheet with parchment paper. Prepare a second baking sheet and parchment paper.

Shape the Dough:

Roll the pastry dough into an 11×16-inch rectangle on a floured surface.

Transfer to the lined baking sheet, dock the surface with a fork, and cover with a second parchment sheet. Add pie weights or dried beans on top.

Bake for 20 minutes, then remove the top sheet and weights. Return to the oven for another 7-9 minutes, until golden brown. Cool for 20 minutes on the pan and transfer to a cooling rack.

Finish the Cream Filling:

Whip the cold heavy cream until medium peaks form.

Gently fold the whipped cream into the chilled pastry cream until combined. Cover and refrigerate for at least 1 hour.

Assemble the Pastries:

Trim the cooled pastry into uniform squares (5 total).

Whisk together the sifted confectioners’ sugar, milk, and vanilla extract to make the icing.

Melt the chocolate in a microwave in 20-second increments, stirring between each until smooth.

Decorate the Pastry Squares:

Spread the vanilla icing over each pastry square. Before the icing sets, pipe lines of melted chocolate across the icing. Use a toothpick to create a chevron pattern by dragging it through the chocolate lines.

Layer the Mille-Feuille:

Fit a piping bag with a large round tip. Pipe 9 dollops of cream on a plain pastry square, then top with another pastry square. Repeat the process with more cream and layers, finishing with a decorated pastry square on top.

Chill and Serve:

Refrigerate the assembled mille-feuille for at least 30 minutes before serving (up to 8 hours).

Slice carefully and serve. Enjoy the delicate layers and indulgent filling!