Preheat your oven to 400°F (200°C). Combine 1 cup of water and 1/2 cup of unsalted butter over medium heat, bringing it to a boil. Add 1 cup of all-purpose flour and stir until a smooth ball forms. Remove from heat, let cool for 5 minutes, then incorporate 4 eggs, one at a time, mixing until glossy.

Pipe the choux pastry into 4-inch lengths on a lined baking sheet. Bake for 25-30 minutes, until puffed and golden-brown. Avoid opening the oven door.

Heat 2 cups of whole milk over medium heat. Whisk together 4 egg yolks and 1/2 cup of granulated sugar until thick. Slowly add heated milk while whisking, then return to the pan and cook until thickened. Fold in 4 oz of dark chocolate and 2 tsp of instant espresso powder.

Poke a small hole in the bottom of each cooled éclair. Fill each with the coffee-infused pastry cream using a piping bag.

Melt 4 oz of dark chocolate with 1/2 cup of heavy cream until smooth. Dip the tops of the éclairs into the glaze and allow excess to drip off before setting on a wire rack.