Preheat oven to 400°F (200°C). In a saucepan, combine water, butter, and salt. Bring to a boil over medium heat.

Remove from heat and add flour all at once. Stir vigorously with a wooden spoon until a smooth dough forms and pulls away from the sides of the pan.

Transfer dough to a bowl and let cool slightly. Add eggs one at a time, mixing well after each addition until the dough is smooth and glossy.

Transfer dough to a piping bag fitted with a large round tip. Pipe 4-inch long eclairs onto a baking sheet lined with parchment paper.

Bake for 25-30 minutes, or until golden brown and puffed up. Let cool completely on a wire rack.

In a saucepan, combine milk, sugar, cornstarch, and salt. Whisk until smooth.

Heat over medium heat, whisking constantly, until the mixture thickens and boils. Boil for 1 minute.

Remove from heat and whisk in egg yolks until smooth. Stir in vanilla extract and butter.

Transfer to a bowl, cover with plastic wrap (pressing it directly onto the surface to prevent a skin from forming), and refrigerate for at least 2 hours.

In a heatproof bowl set over a simmering pot of water (or in the microwave in 30-second intervals, stirring in between), melt the chocolate and butter together until smooth.

Stir in the heavy cream until well combined.

Using a knife or piping bag, poke a hole in the bottom of each eclair. Fill each eclair with pastry cream.

Dip the tops of the eclairs in the chocolate glaze.

Let the glaze set before serving.