Preheat your oven to 400°F (200°C).

In a medium saucepan, combine 1 cup of water with ½ cup of unsalted butter and bring to a rolling boil.

Quickly add 1 cup of all-purpose flour all at once, stirring vigorously until a cohesive dough forms.

Add 4 large eggs to the dough one at a time, mixing thoroughly after each addition.

Grease a baking dish and spread the pastry dough into it, smoothing the surface.

Bake in the preheated oven for 20-25 minutes until golden brown and puffy.

Remove the pastry from the oven and allow it to cool in the baking dish.

In another saucepan, heat 2 cups of whole milk, ½ cup of granulated sugar, and 2 tablespoons of cornstarch until thickened.

Stir in 2 teaspoons of vanilla extract and let cool to room temperature.

Carefully split the pastry into two equal layers and spread the custard mixture over one layer.

Place the second layer of pastry on top and chill for at least 1 hour before serving.