Make the Choux Pastry:

Preheat the oven to 400°F (200°C).

In a saucepan over medium heat, combine water, butter, and salt. Bring to a rolling boil.

Remove from heat and add flour all at once. Stir vigorously until the dough forms a smooth ball and pulls away from the pan.

Let the dough cool for 3 minutes. Beat in eggs one at a time, mixing well after each, until the dough is smooth and shiny.

Transfer the dough to a pastry bag fitted with a small round tip and pipe 3-inch logs onto a parchment-lined baking sheet, spacing them 2 inches apart.

Bake at 400°F for 10 minutes, then reduce heat to 350°F (175°C) and bake for an additional 20-25 minutes until golden and puffed. Cool completely on a wire rack.

Prepare the Coffee Pastry Cream:

In a saucepan, combine milk, cream, and half of the sugar. Heat until just simmering.

In a bowl, whisk the egg yolks, remaining sugar, flour, espresso powder, vanilla, and salt until smooth.

Slowly whisk the hot milk mixture into the yolks to temper, then return to the saucepan. Cook over medium heat, whisking constantly, until thickened and bubbling.

Remove from heat, strain through a fine mesh sieve, cover the surface with plastic wrap to prevent a skin, and chill until cold.

Make the Mocha Glaze:

Place the dark chocolate in a heatproof bowl.

In a small saucepan, heat the cream with espresso powder until steaming. Pour over the chocolate and let sit for 1 minute, then stir until smooth.

Stir in the butter until fully melted. If the glaze is too thin, whisk in powdered sugar 1 tablespoon at a time until the desired consistency is reached.

Assemble the Éclairs:

Fill a clean pastry bag with the chilled pastry cream and fit it with a small round tip. Poke a hole in the bottom of each choux shell and pipe in the cream until just full.

Dip the top of each éclair into the mocha glaze, letting excess drip off. Place on a rack until the glaze sets.

Serve:

Arrange the éclairs on a platter and serve within 2 hours for best texture.