• Step 1: Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper. Unfold the puff pastry sheet and place it on the prepared baking sheet. Bake for 15-18 minutes, or until the pastry is golden brown and puffed. Allow it to cool completely.
• Step 2: In a medium saucepan, whisk together the milk, granulated sugar, and cornstarch. Place the pan over medium heat and cook, stirring constantly, until the mixture thickens and comes to a boil, about 5-7 minutes.
• Step 3: In a separate bowl, whisk the egg yolks. Slowly pour about 1/2 cup of the hot milk mixture into the egg yolks, whisking constantly to temper the eggs. Pour the egg mixture back into the saucepan with the remaining milk mixture. Cook, stirring constantly, until the custard thickens enough to coat the back of a spoon, about 2-3 minutes.
• Step 4: Remove the custard from the heat and stir in the vanilla extract. Transfer the custard to a clean bowl and cover the surface with plastic wrap to prevent a skin from forming. Refrigerate for at least 2 hours, or until completely chilled.
• Step 5: Once the puff pastry and custard are cooled, use a serrated knife to carefully slice the puff pastry into three even layers.
• Step 6: Place one layer of puff pastry on a serving platter. Spread half of the chilled custard evenly over the top. Top with a second layer of puff pastry and the remaining custard. Finish with the final layer of puff pastry.
• Step 7: Refrigerate the assembled Napoleon cake for at least 30 minutes to allow the layers to set.
• Step 8: Just before serving, dust the top of the Napoleon cake generously with powdered sugar.