1. Preheat the oven to 400°F and line a baking sheet with parchment paper.

2. In a saucepan, bring the water, butter, and salt to a boil over medium heat.

3. Add the flour all at once and stir quickly until a smooth dough forms and pulls away from the sides of the pan.

4. Remove from heat and let the dough cool for 5 minutes.

5. Beat in the eggs one at a time until the dough is smooth, glossy, and pipeable.

6. Transfer the dough to a piping bag and pipe long éclair shapes onto the prepared baking sheet.

7. Bake for 30 to 35 minutes, or until puffed and golden brown. Let the shells cool completely.

8. To make the pastry cream, heat the milk in a saucepan until warm but not boiling.

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

10. Gradually whisk the warm milk into the egg mixture, then return everything to the saucepan.

11. Cook over medium heat, whisking constantly, until the pastry cream thickens.

12. Remove from heat and stir in the butter and vanilla extract. Cover and chill until fully cooled.

13. Fill the cooled éclair shells with the chilled pastry cream using a piping bag fitted with a small tip.

14. For the glaze, melt the chopped chocolate, butter, and heavy cream together until smooth.

15. Dip the tops of the filled éclairs into the chocolate glaze and let them set before serving.