In a medium saucepan, combine 1 cup filtered water and ½ cup unsalted butter over medium heat, bringing it to a rolling boil. Once boiling, remove the pan from heat and quickly stir in 1 cup of all-purpose flour until a smooth paste forms; this may take around 2-3 minutes. Allow the dough to cool for about 10 minutes before adding 4 large eggs, mixing until fully incorporated and glossy.

Preheat your oven to 400°F (200°C). Transfer the choux pastry to a piping bag fitted with a large round tip. On a lined baking sheet, pipe 4-inch long strips, ensuring space between each for puffing. Bake in the preheated oven for 25-30 minutes until the éclairs are puffed and golden brown; they should feel light and airy to the touch. Once done, remove from the oven and allow them to cool completely on a wire rack.

In a saucepan, heat 2 cups of milk over medium heat until steaming, but not boiling. Meanwhile, in a mixing bowl, whisk together 4 egg yolks, ½ cup granulated sugar, and ¼ cup all-purpose flour until smooth. Gradually whisk in the hot milk to temper the yolks, then return the mixture to the saucepan, cooking over low heat until thickened, about 5-7 minutes. Stir in 2 tablespoons of instant espresso powder and 1 cup of heavy cream, then let it cool before refrigerating the pastry cream.

Once the choux pastries have cooled and your coffee-infused pastry cream is chilled, use a piping bag fitted with a narrow tip to fill each éclair. Insert the tip into the bottom of each pastry and gently squeeze until filled, feeling the slight resistance. This will ensure your éclairs are generously stuffed with the delicious coffee-infused pastry cream delight!

To prepare the mocha glaze, melt 4 ounces of dark chocolate in a heatproof bowl set over a saucepan of simmering water. Stir in ½ cup of warmed heavy cream until smooth and glossy. Once your chocolate mixture reaches a drizzle-like consistency, remove from heat and let it cool slightly before using.

Dip the filled éclairs' tops into the mocha glaze, ensuring a generous coating, or use a spoon to drizzle the glaze for a more decorative approach. Let the glaze set on a wire rack before serving. These Coffee-Infused Pastry Cream Delights are best served fresh, showcasing their crispy shells and rich creamy centers, perfect for impressing your guests!