Preheat the oven to 400°F. Line a baking sheet with parchment paper.
In a medium saucepan, combine water, butter, and salt. Bring to a boil over medium heat. Once boiling, remove from heat and stir in flour until a dough forms.
Return to low heat and stir for 1-2 minutes until the dough pulls away from the sides of the pan.
Transfer the dough to a mixing bowl. Allow to cool for 5 minutes. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.
Using a piping bag fitted with a large round tip, pipe 4-inch long strips onto the prepared baking sheet, leaving space between each.
Bake for 25-30 minutes or until golden brown and puffed. Remove from the oven and let cool completely.
For the mocha pastry cream, whisk together milk, sugar, cocoa powder, cornstarch, and salt in a saucepan over medium heat.
In a separate bowl, whisk egg yolks. Once the milk mixture begins to simmer, slowly pour half into the egg yolks, whisking constantly.
Return the mixture to the saucepan and cook until thickened, about 2-3 minutes. Remove from heat and stir in butter, instant coffee granules, and vanilla extract. Let cool completely.
Once the éclairs are cool, fill a piping bag with the mocha pastry cream and inject it into each éclair from both ends.
For the chocolate glaze, melt chocolate and butter together in a microwave-safe bowl in 30-second intervals, stirring until smooth.
Dip the tops of each filled éclair into the glaze and let set on a wire rack.
Notes
For a stronger coffee flavor, increase the amount of instant coffee granules in the pastry cream.
Add a sprinkle of sea salt on top of the chocolate glaze for a delightful sweet and salty contrast.