Step 1: Prepare the Puff Pastry
Preheat your oven to 400°F (200°C). Line two baking sheets with parchment paper. Unfold the puff pastry on a lightly floured surface. If needed, roll it slightly to smooth out fold lines. Cut the pastry into three equal rectangles, approximately 10×4 inches each. Place them on the prepared baking sheets, leaving space between each piece. Prick the pastry all over with a fork (this prevents excessive puffing). Place another piece of parchment on top of each pastry rectangle, then place another baking sheet on top to weigh them down. This creates flat, even layers rather than puffy, uneven ones.

Step 2: Bake the Pastry
Bake for 20 minutes with the weight on top. Remove the top baking sheet and parchment, sprinkle each pastry layer generously with granulated sugar, and continue baking for 8-10 more minutes until deep golden brown and caramelized. The sugar will create a beautiful caramel crust. Transfer to a wire rack and let cool completely. These can be made a day ahead and stored in an airtight container.

Step 3: Make the Pastry Cream
In a medium saucepan, heat the milk with ¼ cup of sugar and the vanilla (if using a vanilla bean, add the seeds and pod to the milk now). Bring just to a simmer, then remove from heat. In a separate bowl, whisk together the egg yolks, remaining ¼ cup sugar, cornstarch, and salt until smooth and pale yellow. This is called tempering. Slowly pour about half of the hot milk into the egg mixture while whisking constantly. This gradually raises the temperature of the eggs without scrambling them.

Step 4: Cook the Pastry Cream
Pour the tempered egg mixture back into the saucepan with the remaining milk. Cook over medium heat, whisking constantly, until the mixture comes to a boil and thickens considerably—this takes about 2-3 minutes. It should be thick enough to coat the back of a spoon and hold its shape. Remove from heat and whisk in the butter pieces and vanilla extract (if you haven’t already used vanilla bean). Whisk vigorously to ensure the butter is fully incorporated.

Step 5: Strain and Chill
Strain the pastry cream through a fine-mesh strainer into a clean bowl to remove any cooked egg bits and ensure a silky texture. Press plastic wrap directly onto the surface of the cream to prevent a skin from forming. Refrigerate for at least 3 hours or overnight until thoroughly chilled and set.

Step 6: Prepare the Fondant Icing
In a medium bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth and pourable but not too thin—it should be about the consistency of thick cream. If using melted white chocolate, whisk it in now for extra shine and stability. Adjust consistency with more powdered sugar (to thicken) or milk (to thin) as needed.

Step 7: Assemble the Mille Feuille
Whisk the chilled pastry cream until smooth and spreadable. Place one pastry layer on your serving platter. Spread half of the pastry cream evenly over it using an offset spatula—aim for about ½-inch thickness. Place the second pastry layer on top and press gently. Spread the remaining pastry cream over this layer. Top with the third and final pastry layer, placing it sugar-side down (save the prettiest side for the top).

Step 8: Ice and Decorate
Pour the fondant icing over the top pastry layer and use an offset spatula to spread it evenly to the edges. Work quickly before it sets. Immediately pipe thin parallel lines of melted dark chocolate across the white icing, spacing them about ½ inch apart. While both are still wet, drag a toothpick or knife perpendicular through the chocolate lines, alternating directions (forward, then backward) to create the classic feathered pattern.

Step 9: Chill and Serve
Refrigerate the assembled Mille Feuille for at least 1 hour to let everything set. This makes cutting much easier. When ready to serve, use a sharp serrated knife to cut into portions, wiping the knife clean between each cut for neat slices. Serve chilled.