Prepare the Choux Pastry: In a saucepan, bring water, milk, butter, and salt to a boil. Remove from heat and quickly stir in the flour. Return to low heat and mix until a smooth ball forms. Transfer dough to a bowl, let it cool slightly, then beat in eggs one at a time until glossy.

Pipe and Bake the Éclair Shells: Preheat oven to 375°F (190°C). Fill a piping bag with the dough and pipe 4-inch logs onto a parchment-lined baking sheet. Bake for 30-35 minutes until golden and puffed. Turn off the oven, crack the door open, and let them cool inside for 10 minutes.

Prepare the Pastry Cream: Heat milk in a saucepan until steaming. In a separate bowl, whisk egg yolks, sugar, and cornstarch until smooth. Slowly pour the hot milk into the egg mixture while whisking, then return to heat and cook until thickened. Remove from heat, stir in vanilla extract, and let cool.

Assemble the Éclairs: Poke small holes in the bottom of each shell and pipe in the pastry cream.

Make the Chocolate Ganache and Finish: Heat heavy cream until warm, then pour over chopped dark chocolate. Stir until smooth. Dip the tops of the éclairs into the ganache and let them set before serving.