Preheat your oven to 400°F (200°C). Lay the thawed puff pastry sheets on parchment-lined baking trays and prick them all over with a fork to prevent bubbling. Bake for 15 to 20 minutes until golden and crisp, then cool completely.

In a saucepan, heat 4 cups of whole milk over medium heat until steaming, do not boil. In a separate bowl, whisk together 6 egg yolks, ½ cup granulated sugar, ¼ cup cornstarch, and 2 tablespoon all-purpose flour until pale and thick.

Gradually pour the hot milk into the egg mixture while whisking continuously. Once combined, return to the saucepan and stir constantly until thickened, about 5 to 7 minutes. Remove from heat, then stir in 3 tablespoon unsalted butter and 1 teaspoon vanilla extract.

Cover the surface of the pastry cream with plastic wrap and chill completely in the refrigerator for at least an hour.

To assemble, place one layer of puff pastry on a serving platter. Spread one third of the cooled pastry cream over it, then add a second layer of pastry and repeat, finishing with the final pastry layer on top.

In a small bowl, whisk together 1 cup powdered sugar, 2 oz milk, and ½ teaspoon vanilla extract until thick and glossy.

Spread the icing evenly over the top layer of the assembled Napoleon Cake. Optionally, drizzle melted dark chocolate over the icing to create a marbled design.

Refrigerate the assembled cake for at least four hours or ideally overnight. For cleaner slices, freeze for about 30 minutes before cutting.