To make this elaborate French dessert, you have to bake sheets of docked puff pastry in between sheet pans to suppress the rise. The result is a crisp wafer-like layer that you then alternate with pastry cream, whipped cream, or jam.
Preheat the oven to 425 degrees F.
To prepare the puff pastry, line a sheet pan with parchment paper. Stack both sheet of puff pastry on top of one another on the parchment. Roll the puff pastry into a 12 x 12-inch square. Cut the Square into three equal pieces. Dock the puff pastry all over with a fork. Transfer the puff pastry to a parchment-lined baking sheet with a second piece of parchment on top, and place in the fridge for 30 minutes to firm.
Remove the puff pastry from the fridge, and bake for 10 minutes. Place a baking sheet on top of the puff pastry and bake for an additional 6 minutes. Then, remove the baking sheet and bake for another 5 to 6 minutes or until the puff pastry is golden brown. Transfer the strips of puff pastry to a cooling rack to come to room temperature.
Once the puff pastry is cool, remove the pastry cream from the fridge. Spread half of the pastry cream on one piece of puff pastry, and repeat with the next layer. Take the third piece of puff pastry and pour the white icing over top. Using a piping bag with a fine tip, pipe lines of chocolate icing lengthwise across the top layer of the puff pastry. Using a tooth pick, drag the chocolate lines horizontally to create a pattern.
Top your napoleon with the decorative top layer. Chill for at least an hour to set before serving.
In a medium bowl, whisk together the powdered sugar, corn syrup, and butter. Then, slowly incorporate the milk until you reach the desired, pourable consistency. Pour a ¼ of the icing into a separate bowl and whisk in the cocoa powder to make the chocolate icing.