Make the Choux Pastry: Preheat oven to 425°F (220°C). In a saucepan, combine butter, water, and salt. Bring to a boil. Add flour all at once and stir vigorously until a dough ball forms. Let cool slightly.

Add eggs one at a time, beating well after each, until smooth and glossy.

Transfer to a piping bag. Pipe 4" strips onto a parchment-lined baking sheet. Bake 10 minutes, then reduce to 375°F (190°C) and bake another 20-25 mins. Cool completely.

Prepare Pastry Cream: Heat milk in a saucepan until steaming. In a bowl, whisk sugar, cornstarch, salt, and egg yolks. Slowly whisk in hot milk. Return to pan and cook over medium heat until thick. Stir in butter and vanilla. Chill completely.

Fill the Éclairs: Use a piping bag with a small tip to fill each éclair shell with pastry cream.

Make Ganache: Heat cream until steaming. Pour over chopped chocolate. Let sit 2 minutes, then stir until smooth.

Dip and Serve: Dip tops of filled éclairs into ganache. Let set. Serve chilled or at room temp.