1.  Preheat the oven to 400°F (200°C) and line a baking tray with parchment paper. 
2.  Combine water, butter, and salt in a saucepan. 
3.  Bring to a boil until the butter is completely melted. 
4.  Add the flour all at once and stir vigorously until a smooth dough forms. 
5.  Cook for 1–2 minutes, stirring constantly, until the dough pulls away from the sides. 
6.  Transfer the dough to a mixing bowl and let it cool slightly. 
7.  Add the eggs one at a time, mixing well after each addition until smooth. 
8.  Transfer the dough to a piping bag fitted with a large round or star tip. 
9.  Pipe long strips onto the prepared baking tray. 
10.  Bake for 25–30 minutes, until puffed and golden. 
11.  Turn off the oven and leave the éclairs inside for 5 minutes with the door slightly open. 
12.  Remove and cool completely. 

Make the Pastry Cream

1.  Heat the milk in a saucepan until steaming but not boiling. 
2.  Whisk the egg yolks, sugar, and cornstarch in a bowl until smooth. 
3.  Slowly whisk the hot milk into the egg mixture. 
4.  Return the mixture to the saucepan and cook over medium heat, stirring constantly, until thick. 
5.  Stir in vanilla and cool completely. 

Fill and Glaze

1.  Make small holes underneath each éclair. 
2.  Pipe the chilled pastry cream into the shells. 
3.  Heat the heavy cream until hot but not boiling. 
4.  Pour it over the chocolate and let stand for 1–2 minutes. 
5.  Stir until smooth and glossy. 
6.  Dip the tops of the éclairs into the chocolate glaze. 
7.  Refrigerate briefly to set the glaze. 
8.  Serve chilled or slightly cool.