In a medium saucepan, bring 1 cup of filtered water and ½ cup of unsalted butter to a rolling boil over medium heat. Add 1 cup of all-purpose flour, stirring until it forms a ball.

Gradually add 4 large eggs, mixing well after each addition until the dough is glossy and pipeable. Preheat oven to 425°F (220°C) and line baking sheets with parchment paper.

Pipe 4-inch-long strips on baking sheets and bake for 20 minutes, then reduce temperature to 375°F (190°C) for 10-15 minutes until golden brown.

Prepare the filling by whipping 1 cup of cold heavy cream with 8 ounces of mascarpone cheese and 2 tablespoons of instant espresso powder until soft peaks form.

Once éclairs are cool, brush each with 1 cup of strong brewed coffee, slice them lengthwise, and fill with mascarpone cream using a piping bag.

Dust with cocoa powder and sift icing sugar on top. Chill for at least 30 minutes before serving.