Preheat your oven to 400°F (200°C).

In a saucepan, combine water, butter, and salt over medium heat until the butter melts.

Bring it to a boil before adding the flour all at once, stirring vigorously until the mixture pulls away from the sides of the pan.

Remove the pan from heat and let it cool for about 5 minutes.

Add eggs one at a time, mixing well after each addition until the dough is smooth.

Transfer the dough to a pastry bag and pipe small mounds onto a parchment-lined baking sheet.

Bake in the preheated oven for 25 minutes until golden brown and puffed.

In a saucepan, whisk together milk, sugar, cornstarch, and egg yolks over medium heat.

Stir continuously until the mixture thickens and bubbles, then remove from heat and stir in vanilla and butter.

Once the puffs have cooled, cut a small slit in the side and fill them with the custard using a pastry bag.