In a medium bowl, vigorously whisk the egg yolks, sugar, and cornstarch until pale and smooth.

Heat the whole milk in a saucepan over medium heat until steaming. Slowly pour half of the hot milk into the yolk mixture while whisking constantly to temper the eggs. Pour the mixture back into the saucepan.

Cook over medium heat, whisking constantly, until thick and bubbling. Remove from heat, whisk in butter and vanilla, press plastic wrap to the surface, and chill for 3 hours until fully set.

Preheat oven to 400°F (200°C). In a saucepan, boil water, milk, butter, sugar, and salt over medium-high heat.

Remove from heat, immediately stir in all the flour until a dough forms. Return to medium heat and cook for 2-3 minutes until a thin film forms on the bottom of the pan.

Transfer the dough to a stand mixer and cool for 5 minutes. Beat in eggs one at a time on low speed until the batter forms a glossy 'V' shape on the paddle.

Pipe 2-inch wide mounds onto parchment-lined baking sheets. Bake at 400°F (200°C) for 15 minutes, reduce heat to 350°F (175°C) without opening the door, and bake for 20-25 more minutes until puffed, cracked, and golden-brown. Poke a hole in each to vent steam and cool completely.

Cut cooled pastry shells horizontally in half. Transfer chilled vanilla cream to a piping bag with a large star tip. Generously pipe a thick swirl of cream onto the bottom halves.

Gently rest the top halves onto the cream. Heavily dust with powdered sugar and serve immediately.