Preheat your oven to 400°F (200°C). In a medium saucepan, combine 1 cup of water, ½ cup unsalted butter, and a pinch of salt, bringing it to a vigorous boil. Once boiling, remove from heat, add 1 cup of all-purpose flour, and stir until a smooth dough forms. Allow it to cool for a few minutes, then beat in 4 eggs one at a time until the mixture is smooth and shiny. Pipe 3-inch logs onto a parchment-lined baking sheet. Bake for 10 minutes, then reduce the temperature to 350°F (175°C) and bake for an additional 20-25 minutes until golden and puffed. Let them cool completely.

In a saucepan, heat 1 cup of whole milk, ½ cup of heavy cream, and ½ cup of granulated sugar, stirring gently until it reaches a simmer. In a mixing bowl, whisk together 4 egg yolks, the remaining sugar, ¼ cup of all-purpose flour, 2 tablespoons of instant espresso powder, 1 teaspoon of vanilla extract, and a pinch of salt until smooth. Gradually whisk the hot milk mixture into the egg yolks. Return the mixture to the saucepan and cook over medium heat, stirring constantly until it thickens and bubbles gently. Strain through a fine mesh sieve and cover with plastic wrap, ensuring it touches the surface to prevent a skin from forming. Chill until completely cold.

In a small saucepan, heat ½ cup of heavy cream along with 1 tablespoon of instant espresso powder until it starts to steam but not boil. Pour this steaming mixture over 4 ounces of chopped dark chocolate in a bowl. Let it sit for a few minutes to melt, then gradually stir until smooth and glossy. If needed, mix in 1 tablespoon of unsalted butter to enhance glossiness. For a thicker glaze, whisk in powdered sugar until you achieve your desired consistency.

Gently poke small holes in the bottom of each cooled choux shell using a skewer or a small knife. Fill a piping bag fitted with a round tip with the cold coffee pastry cream. Carefully fill each shell with the cream until you feel them slightly puff up, ensuring not to overfill to prevent sogginess. Dip the tops of each filled éclair into the warm mocha glaze, allowing any excess to drop off before placing them on a wire rack to set.

Arrange your stunning Mocha Éclairs on a platter, showcasing their glossy tops. For the best texture and flavor, serve them within 2 hours of assembly. Pair with freshly brewed coffee or espresso to elevate your dessert experience.