Preheat your oven to 400°F (200°C) and line two baking sheets with parchment paper. In a medium saucepan, bring water and butter to a boil over medium heat. Stir in flour and salt until a smooth dough forms. Remove from heat and let it cool for about 5 minutes before adding the eggs, one at a time, mixing well after each addition until glossy.

Transfer the choux pastry into a piping bag fitted with a large round tip. Pipe 4-inch logs spaced 3 inches apart on the prepared baking sheets. Smooth any peaks with a wet finger. Bake in the preheated oven for 30-35 minutes until golden brown and hollow when tapped.

In a saucepan, heat milk over medium heat until steaming. Whisk egg yolks, sugar, cornstarch, and salt in a bowl. Gradually pour the hot milk into the mixture, whisking continuously. Return it to the saucepan and cook until thickened, about 2-3 minutes. Stir in vanilla extract and butter until smooth, then allow to cool completely.

Once the eclair shells are cooled, use a small paring knife to cut a small hole in the side of each eclair. Insert a piping bag filled with the pastry cream into the hole and gently squeeze to fill each until slightly plump.

In a microwave-safe bowl, combine dark chocolate, heavy cream, butter, and corn syrup. Microwave in 30-second intervals until melted and smooth. Dip the tops of filled eclairs in the chocolate glaze and let excess drip off before placing them back.