Bring water and butter to a boil in a saucepan over medium heat, stirring until the butter melts fully (about 3 minutes).

Remove from heat and add flour in one go, stirring vigorously until a smooth, slightly sticky dough forms.

Return pan to low heat, cook dough for 1-2 minutes until it leaves a thin film on the pan’s bottom.

Let dough cool 2-3 minutes, then beat in eggs one at a time until mixture is glossy and elastic.

Transfer dough to a piping bag fitted with a large round tip and pipe 4-inch logs on a parchment-lined baking sheet.

Bake at 400°F (200°C) for 10 minutes, then reduce oven to 350°F (175°C) and bake 15-18 minutes until golden brown.

Remove éclairs from oven and cool on a wire rack for about 20 minutes to ensure crisp shells.

In a medium bowl, whisk together egg yolks, sugar, and cornstarch until the mixture is pale, silky smooth, and lump-free.

Heat milk until steaming but not boiling, then slowly whisk it into the yolk mix to prevent curdling.

Return mixture to saucepan and cook over medium-low heat for 5-7 minutes, stirring constantly until it thickens and coats the back of a spoon.

Remove from heat, stir in chopped dark chocolate until fully melted and creamy, then cool slightly before piping into shells.

Warm heavy cream in a small saucepan until tiny bubbles form around edges (about 2 minutes).

Pour hot cream over chopped bittersweet chocolate and butter in a heatproof bowl, let stand 1 minute, then stir gently until glossy.

Dip tops of cooled éclairs into the glaze, letting excess drip off, and transfer to a wire rack to set.