Step 1: Preheat your oven to 400°F (200°C). Unroll two sheets of thawed puff pastry onto separate parchment-lined baking sheets. Using a fork, generously prick the entire surface of each pastry sheet.
Step 2: Cover each sheet of pastry with another piece of parchment paper, and then place another baking sheet on top. Bake for 15 minutes.
Step 3: Remove the top baking sheets and parchment. Sprinkle one pastry sheet with about 1-2 tablespoons of granulated sugar. Return both sheets to the oven and continue baking for another 10-15 minutes, or until deeply golden brown and caramelized.
Step 4: Once baked, carefully remove the pastry from the oven. While still warm, use a sharp serrated knife to trim the edges and cut each sheet into three equal rectangles. Let them cool completely on a wire rack.
Step 5: In a medium saucepan, combine 2 cups whole milk, 1/2 cup granulated sugar (from the total), and the scraped seeds and pod of one vanilla bean. Heat over medium heat until simmering, stirring occasionally.
Step 6: In a separate bowl, whisk together 4 large egg yolks, 1/4 cup granulated sugar, and 1/4 cup cornstarch until smooth and pale.
Step 7: Once the milk mixture is simmering, slowly temper the hot milk into the egg yolk mixture, whisking continuously. Pour the tempered mixture back into the saucepan.
Step 8: Cook over medium heat, whisking constantly and vigorously, until the cream thickens significantly and comes to a boil. Boil for 1-2 minutes, still whisking, to ensure the cornstarch is fully cooked.
Step 9: Remove from heat and stir in 2 tablespoons of unsalted butter until melted and incorporated. Strain the pastry cream through a fine-mesh sieve into a clean bowl.
Step 10: Press a piece of plastic wrap directly onto the surface of the cream. Chill in the refrigerator for at least 4 hours, or preferably overnight, until thoroughly cold and firm.
Step 11: Once the pastry cream is chilled, give it a good whisk to loosen it up and make it smooth. Transfer the cream to a piping bag fitted with a large plain round or star tip.
Step 12: Take one cooled puff pastry rectangle and pipe an even layer of vanilla pastry cream over its surface.
Step 13: Gently place a second puff pastry rectangle on top of the cream layer, pressing down very lightly. Pipe another layer of cream.
Step 14: Top with the third puff pastry rectangle, pressing gently to create a stable stack. Repeat this process for the remaining pastry and cream.
Step 15: Melt 1/2 cup (about 85g) good quality dark or semi-sweet chocolate chips or chopped chocolate in a microwave-safe bowl in 30-second intervals, stirring after each, until smooth and fully melted.
Step 16: Transfer the melted chocolate to a small piping bag with the tip snipped very tiny, or a zip-top bag with a corner snipped. Drizzle decoratively over the top layer of each mille feuille.
Step 17: Chill briefly to set the chocolate before serving.