In a saucepan, combine water, unsalted butter, and a pinch of salt. Bring to a boil, then stir in all-purpose flour until the mixture is smooth. Remove from heat and let it cool for about 5 minutes.

Add room temperature eggs to the choux mixture one at a time. Mix until smooth and glossy; it should form a thick batter that holds its shape.

Pipe the choux pastry onto a lined baking sheet to create two equal layers. Bake in a preheated oven at 400°F (200°C) for around 25-30 minutes or until they are golden brown and puffed.

In a medium saucepan, whisk together milk, granulated sugar, cornstarch, and vanilla extract. Cook over medium heat, whisking continuously, until the mixture thickens and reaches a creamy consistency.

Once the pastry layers cool, spread the rich vanilla custard filling generously between them and stack the layers.

Cover the assembled cake with plastic wrap and refrigerate for at least 2 hours, allowing the filling to set before serving.