In a medium saucepan, combine water and butter. Bring to a boil over medium heat.

Once boiling, add the flour and salt all at once. Stir vigorously with a wooden spoon until the mixture forms a ball and pulls away from the sides of the pan, about 1-2 minutes.

Remove from heat and let cool for about 5 minutes.

Add eggs one at a time, mixing well after each addition until smooth and glossy. The batter should be thick but pipeable.

Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.

Transfer the choux pastry to a piping bag fitted with a large round tip. Pipe 4-inch long strips onto the prepared baking sheet, leaving space between each.

Bake for 20-25 minutes, or until puffed and golden brown. Do not open the oven door during baking.

Once done, remove from the oven and let cool completely on a wire rack.

In a medium saucepan, heat the milk over medium heat until steaming but not boiling.

In a bowl, whisk together sugar, cornstarch, and egg yolks until smooth.

Slowly pour the hot milk into the egg mixture while whisking continuously to temper the eggs.

Return the mixture to the saucepan and cook over medium heat, whisking constantly until thickened, about 3-5 minutes.

Remove from heat and stir in vanilla extract and butter until smooth. Transfer to a bowl, cover with plastic wrap (pressing it directly onto the surface), and chill in the refrigerator until set.

In a small saucepan, heat the heavy cream over medium heat until it begins to simmer.

Remove from heat and add the chopped chocolate. Let it sit for a few minutes, then stir until smooth and glossy.

Once the éclairs are cool, cut them in half lengthwise. Fill the bottom half with the chilled pastry cream using a piping bag or spoon.

Dip the top half in the chocolate ganache or drizzle it over the filled éclairs. Allow the ganache to set before serving.