To make the pastry cream, put milk into a medium saucepan. Scrape seeds from vanilla bean, add seeds and pod to milk, and bring to a simmer over medium heat. Remove pan from heat, cover, and set aside to steep for 1 hour. Strain milk into a bowl, discarding pod. Return milk to saucepan and bring to a simmer again over medium heat.
Meanwhile, beat egg yolks, 10 tbsp. of the granulated sugar, and cornstarch together in a mixing bowl with an electric mixer on medium speed until pale yellow and mixture falls from beaters in thick ribbons, 3-5 minutes. Gradually add 1 cup of the hot milk to egg yolk mixture, whisking constantly; then gradually add egg yolk-milk mixture back into hot milk in saucepan. Cook over medium heat, stirring constantly with a wooden spoon, until custard is thick enough to coat the back of a spoon, about 5 minutes. Transfer to a medium bowl and stir constantly until custard is very warm (about 125° on an instant-read thermometer), 5-10 minutes. Add butter, 1 tbsp. at a time, whisking well after each addition. Cover surface of pastry cream with plastic wrap to prevent a skin from forming and refrigerate until ready to use.
Preheat oven to 375°. Put 1 sheet of puff pastry on a parchment paper-lined baking sheet, cover with another sheet of parchment, and refrigerate until chilled, 20-30 minutes. Put another baking sheet directly on top of parchment-covered pastry to weight it, then bake until pastry begins to turn brown, 10-15 minutes. Remove baking sheet and parchment from top of pastry. Sprinkle pastry with 1 tbsp. granulated sugar, then bake, uncovered, until pastry is golden brown, 6-8 minutes more. Transfer pastry to a clean surface and remove parchment from bottom. Set aside to cool. Repeat process with remaining 1 sheet puff pastryand 1 tbsp. sugar. Using a serrated knife, cut each pastry into twelve 4" x 2" rectangles, making 24 rectangles in all.
To assemble the napoleons, spoon 2 tbsp. of the pastry cream down center of each sugared side of 8 pastry rectangles and stack another pastry rectangle on top. Repeat process with remaining pastry cream and pastry rectangles. Sift confectioners' sugar over napoleons.