Prepare the Puff Pastry:

1. Preheat your oven to 400°F (200°C). Line two large baking sheets with parchment paper.
2. Unfold the thawed puff pastry sheets on a lightly floured surface. If needed, gently roll each sheet to smooth out any creases and create an even rectangle approximately 10×12 inches.
3. Transfer each pastry sheet to a prepared baking sheet. Using a fork, dock the entire surface of each sheet by pricking it all over at ½-inch intervals. This prevents the pastry from puffing up unevenly.
4. Place a second sheet of parchment paper on top of each pastry sheet, then place another baking sheet on top to weight it down. This technique ensures flat, even layers perfect for stacking.
5. Bake for 20-25 minutes until deep golden brown and completely crisp. Remove the top baking sheets and parchment carefully, then return pastry to oven for 2-3 minutes to crisp up the top surface if needed. Transfer to cooling racks and cool completely.
6. Once cooled, carefully trim the edges of each pastry sheet to create clean rectangles. Cut each sheet into thirds lengthwise, creating 6 strips total (you’ll need 3 strips for one large Napoleon or use all 6 for two Napoleons).

Make the Pastry Cream:

7. In a medium saucepan, heat the milk with ¼ cup of sugar over medium heat, stirring occasionally until it just begins to steam and small bubbles form around the edges. Do not boil.
8. While the milk heats, whisk together the egg yolks, remaining ¼ cup sugar, cornstarch, and salt in a medium bowl until smooth and pale yellow, about 1-2 minutes.
9. When the milk is hot, slowly pour about ½ cup of the hot milk into the egg mixture while whisking constantly. This tempers the eggs and prevents them from scrambling.
10. Pour the tempered egg mixture back into the saucepan with the remaining hot milk, whisking constantly.
11. Cook over medium heat, whisking continuously and making sure to scrape the bottom and corners of the pan, until the mixture thickens considerably and begins to bubble, about 3-5 minutes. Continue cooking and whisking for 1-2 minutes more to cook out the starchy taste.
12. Remove from heat and whisk in the butter and vanilla extract until the butter melts and is fully incorporated.
13. Pour the pastry cream through a fine-mesh strainer into a clean bowl to remove any lumps and ensure silky-smooth texture.
14. Press plastic wrap directly onto the surface of the cream to prevent a skin from forming. Refrigerate for at least 2 hours or until completely chilled and set. Before using, whisk the chilled pastry cream until smooth.

Assemble the Napoleons:

15. Place one pastry layer on a serving platter or cutting board. Using an offset spatula, spread half of the chilled pastry cream evenly over the surface, leaving about ¼ inch border around the edges.
16. Carefully place the second pastry layer on top of the cream, pressing down very gently to help it adhere.
17. Spread the remaining pastry cream over the second layer in the same manner.
18. Place the third and final pastry layer on top, pressing down gently. The pastry should be level and stable.

Prepare the Icing and Decorate:

19. In a medium bowl, whisk together the powdered sugar, 2 tablespoons milk, and vanilla extract until smooth and pourable but not too thin. Add an additional tablespoon of milk if needed to reach the consistency of thick honey.
20. Pour the white icing over the top pastry layer and use an offset spatula to spread it evenly to the edges.
21. Melt the chocolate in a microwave-safe bowl in 20-second intervals, stirring between each, until smooth.
22. Transfer the melted chocolate to a small piping bag or a small zip-top bag with a tiny corner snipped off. Pipe thin parallel lines of chocolate across the width of the white icing, spacing them about ¾ inch apart.
23. Working quickly before the icing sets, drag a toothpick or skewer through the chocolate lines perpendicular to them, alternating directions (pull toward you for one line, then push away for the next) to create the classic feathered pattern.
24. Refrigerate the assembled Napoleon for 30 minutes to set the icing before slicing.
25. Use a sharp serrated knife to cut into individual portions. Wipe the knife clean between cuts for neat slices. Serve chilled.