Put milk into a medium saucepan.

Scrape seeds from the vanilla bean and add seeds and pod to milk.

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 the pod.

Return milk to the saucepan and bring to a simmer over medium heat.

Meanwhile, beat egg yolks, 10 tbsp. of granulated sugar, and cornstarch together in a mixing bowl with an electric mixer on medium speed until pale yellow and thick, about 3-5 minutes.

Gradually add 1 cup of the hot milk to the egg yolk mixture, whisking constantly.

Then gradually add the egg yolk-milk mixture back into the hot milk in the saucepan.

Cook over medium heat, stirring constantly with a wooden spoon, until the custard is thick enough to coat the back of a spoon, about 5 minutes.

Transfer to a medium bowl and stir constantly until very warm (about 125°F on an instant-read thermometer), 5-10 minutes.

Add butter, 1 tbsp. at a time, whisking well after each addition.

Cover the surface of the pastry cream with plastic wrap to prevent a skin from forming and refrigerate until ready to use.

Preheat oven to 375°F (190°C).

Place 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 the parchment-covered pastry to weight it down.

Bake until the pastry begins to turn brown, about 10-15 minutes.

Remove the top baking sheet and parchment paper.

Sprinkle the pastry with 1 tbsp. granulated sugar, then bake, uncovered, until golden brown, about 6-8 minutes more.

Transfer to a clean surface and remove the parchment from the bottom.

Set aside to cool.

Repeat the process with the remaining sheet of puff pastry and 1 tbsp. sugar.

Using a serrated knife, cut each pastry sheet into twelve 4" x 2" rectangles, making 24 rectangles in total.

Spoon 2 tbsp. of the pastry cream down the center of each sugared side of 8 pastry rectangles and stack another pastry rectangle on top.

Repeat the process with the remaining pastry cream and pastry rectangles.

Sift confectioners' sugar over the assembled napoleons.