Bake the Puff Pastry Layers:

1. Preheat oven:
   • Preheat your oven to 400°F (200°C).
2. Prepare the pastry:
   • Roll out the puff pastry into thin sheets, approximately 1/8 inch thick. Cut into rectangles or circles to fit your desired cake size. You’ll need about 10–12 layers total.
3. Bake:
   • Place the pastry layers on a parchment-lined baking sheet. Prick each sheet with a fork to prevent puffing. Bake for 12–15 minutes, or until golden brown and crispy. Let cool completely.

Make the Custard Filling:

4. Heat the milk:
   • In a medium saucepan, heat the milk over medium heat until it just begins to simmer.
5. Whisk the yolks and dry ingredients:
   • In a separate bowl, whisk together the egg yolks, sugar, and flour until smooth.
6. Temper the eggs:
   • Slowly pour a small amount of the hot milk into the yolk mixture, whisking constantly to prevent curdling. Gradually add the tempered yolks back into the saucepan of milk.
7. Thicken the custard:
   • Cook the mixture over medium-low heat, stirring constantly, until thickened. Remove from heat and stir in the butter and vanilla extract. Let cool to room temperature.

Assemble the Napoleon Cake:

8. Layer the cake:
   • Spread a generous amount of custard filling between each puff pastry layer, stacking them carefully. Reserve a small amount of custard for the top.
9. Finish and chill:
   • Spread the remaining custard or Cool Whip on top. Crush one of the pastry sheets into crumbs and sprinkle over the cake for decoration. Refrigerate for at least 1 hour (preferably overnight) to allow the layers to soften slightly.
10. Serve:
    • Slice carefully and serve chilled.