• In a small dish, stir the gelatin and water together to combine. There should be just enough water to moisten the gelatin, creating a rubbery paste. Set aside.
• Add the egg yolks and cornstarch to a medium-size bowl and beat with a wire whisk to combine. Set the bowl on a kitchen towel somewhere close to the stovetop. (This will keep the bowl from sliding around on your counter when you whisk in the hot milk.)
• Set a 1 or 2 cup heatproof measuring cup next to the stovetop, preferably one with a pourable spout.
• Add the milk, sugar, and salt to a 3-quart or larger heavy-bottomed saucepan and stir to combine. Cook over medium heat, stirring frequently, just until it barely begins to boil. Remove the pan from the heat.
• Pour 1 & 1/2 cups (approximatly) of the hot milk into the glass measuring cup. Slowly pour the hot milk from the measuring cup into the egg yolks, pouring slowly, and whisking constantly. Then, pour everything back into the saucepan and set it over low heat. (You want to continue to heat the eggs slowly so they won’t scramble.)
• Cook, stirring constantly until the custard is thick and begins to boil.
• As soon as thickens to the consistency of pudding and barely begins to boil, remove from the heat and whisk for about 20 seconds longer. Pour the pastry cream into a bowl.
• Break the gelatin into small pieces, dropping them onto the hot pastry cream. Let stand for about 20 seconds to let the gelatin to melt, then stir until the gelatin is completely incorporated. Stir in the vanilla.
• Cover with plastic wrap, pressing the wrap over the surface of the pastry cream. Refrigerate for at least 3 hours and up to 48 hours. (*At this point, the pastry cream can be frozen for up to 3 months.)
• Using an electric mixer, whip the heavy cream until stiff peaks form. (*See note below if you are going to use this pastry cream in something that will be baked.)
• Remove the cooled pastry cream from the refrigerator and add to the whipped cream. Beat on medium speed with an electric mixer to combine, about 1 minute. The pastry cream is now ready to use in any recipe.

Prepare the puff pastry:

• NOTE: I’ve heard from a few readers that the long bake time in this recipe has resulted in overdone puff pastry. After making this recipe many, many times, I can attest that the long bake time works well in my kitchen every single time. I don’t have a good explanation for why the bake time is sometimes too long for others. It’s a mystery I hope one day to solve. ***UPDATE! I think one reader might have solved this mystery. It has to do with how thawed your puff pastry is. I’ve added her exact words below, in the notes.
• In the mean time, to prevent overcooking, peek under the top baking sheet and check the pastry after 15 minutes of baking. If it’s already getting brown, go ahead and remove the top baking sheet and bake for another 5-10 minutes until golden brown. The goal is to end up with puff pastry that is only slightly puffed, golden brown, and flaky.
• Thaw the puff pastry according to the package directions.
• Preheat the oven to 400 degrees and cover a baking sheet with parchment paper.
• Unfold both sheets of pastry and divide each into thirds by cutting along the fold lines, giving you 6 rectangles of pastry. Lay the rectangles on the parchment-covered baking sheet, spacing them about 1/4-inch apart.
• Cover the pastry with another sheet of parchment paper and lay another baking sheet on top. This will prevent the puff pastry from rising too much and give the pastry nice, flaky layers.
• Bake for 35-40 minutes. Remove the top baking sheet and the top layer of parchment and bake for another 5-10 minutes, until the pastry is golden brown.
• Remove from the oven and let cool completely before assembly.

ASSEMBLE THE NAPOLEONS:

• Lay one sheet of baked puff pastry on a serving dish and spread about a 1/2-inch thick layer of pastry cream evenly over the top. Layer another rectangle of pastry over the cream, repeat with another 1/2-inch thick layer of pastry cream and top with a third pastry rectangle. Press on the top of the Napoleon slightly just to evenly distribute the layers. Repeat with the remaining pastry rectangles and pastry cream. Cover the Napoleons with plastic wrap and refrigerate for at least 2 hours.
• Dust the tops of each Napoleon with a thick layer of powdered sugar right before serving.
• Slice each Napoleon into 6 slices, using a serrated knife. *TIP: Hold on to the sides of each pastry layer as you slice through it, sawing gently with the knife, allowing the knife to do the work of cutting through the fragile layers of pastry without pressing down too hard.