Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper and fit a piping bag with a large round tip. Keep ingredients handy.

In a medium saucepan over medium heat, combine water, butter, and salt. Bring to a boil, remove from heat, stir in flour until a smooth dough forms.

Transfer dough to a bowl; beat in eggs one at a time until glossy, smooth, and pipeable—about 3 minutes. The dough should fall slowly from the spoon.

Pipe 4-inch logs onto the sheet; leave space to expand. Bake at 375°F for 20–25 minutes until golden brown and puffy. Avoid opening the oven door.

Turn off oven and crack door open for 10 minutes. Remove éclairs and cool completely on a wire rack—this keeps them crisp and hollow.

In a saucepan, simmer whole milk until tiny bubbles form around edges—do not boil. Meanwhile, whisk sugar, yolks, and cornstarch until pale.

Slowly pour hot milk into yolk mixture, whisking constantly. Return to stove; cook over medium heat, whisking until thick and smooth—about 2 minutes.

Remove from heat, stir in vanilla extract. Transfer pastry cream to a bowl, cover surface with plastic wrap, and chill at least 1 hour.

Place chopped semi-sweet chocolate and butter in a heatproof bowl. Warm heavy cream until steaming; pour over chocolate, let sit 1 minute, stir until glossy.

Trim éclair tops, pipe chilled pastry cream inside. Dip tops into warm glaze, place back on rack. Let set for 10 minutes before serving.