Dice the butter into 1-inch (2.5cm) cubes and freeze for 10 minutes.

In a large bowl, combine the bread flour, all-purpose flour, salt, and sugar.

Add the butter cubes to the flour mixture. Work the dough with your hands until all the butter is coated in flour and large chunks of butter remain visible.

Add a couple of tablespoons of water and mix until absorbed. Repeat this process until the dough forms and no dry flour patches remain. Do not knead—press and squeeze the mixture gently to bring it together. You may need slightly more or less water to hydrate the flour, so add water gradually until the dough comes together.

Place the dough on a floured surface and roll it into a rectangle. Fold it in half lengthwise, then fold the top and bottom edges toward the center, leaving a small gap between them. Fold in half again to form a "book" shape. Wrap in plastic wrap and refrigerate until firm.

Heat the milk in a saucepan until it just begins to simmer, then remove from heat.

In a bowl, whisk together the sugar, egg yolks, cornstarch, vanilla extract, and a pinch of salt until smooth.

Slowly pour ⅓ of the hot milk into the egg yolk mixture, whisking vigorously to prevent curdling.

Pour the tempered mixture back into the saucepan with the remaining milk. Heat over medium heat, stirring constantly.

Once it begins to boil, cook for 1–2 minutes, until thickened to a pudding-like consistency with no raw starchy taste.

Remove from heat and whisk in the butter until fully melted and incorporated.

Transfer the mixture to a clean bowl and cover with plastic wrap pressed directly against the surface to prevent a skin from forming. Refrigerate until cold.

Preheat the oven to 400F (205C). Line a baking sheet with parchment paper.

Divide the chilled dough into two equal portions. Roll out one portion on a floured surface into a rectangle and cut it into 12 rectangles, each 2x4 inches (5x10cm).

Arrange the rectangles on the prepared baking sheet, leaving space between them. Cover with another sheet of parchment paper and place a second baking sheet on top to weigh them down.

Bake for 12–15 minutes, or until golden brown. Transfer to a cooling rack and let cool completely.

Repeat with the second portion of dough so you have 24 rectangles in total.

Whip the heavy cream until stiff, shiny peaks form.

Using the same mixer, whip the chilled pastry cream until smooth.

Gently fold one-third of the whipped cream into the pastry cream with a rubber spatula, then fold in the remaining whipped cream until fully combined. Transfer to a piping bag fitted with round nozzle.

Repeat with the remaining puff pastry and cream to assemble 8 Mille-Feuille pastries.