In a bowl, mix 50g of softened unsalted butter, 50g of light brown sugar, and 50g of all-purpose flour until a smooth dough forms. Roll this mixture between two sheets of parchment paper until it's about 2mm thick. Once done, place it in the refrigerator to chill for at least 30 minutes.

In a saucepan, combine 125ml of water (or half water and half milk), 60g of unsalted butter, ¼ teaspoon of salt, and 1 teaspoon of granulated sugar (if using). Bring the mixture to a gentle boil over medium heat. Once boiling, remove from heat and quickly stir in 75g of sifted all-purpose flour until a ball forms.

After cooling, transfer the dough into a mixing bowl and gradually add 2 room temperature eggs, beating well after each addition until the mixture is smooth and glossy.

Transfer the choux dough into a piping bag fitted with a plain round tip. On a parchment-lined baking sheet, pipe mounds of dough, about 2 inches apart—aim for a nice round shape.

Place the baking sheet in the preheated oven and bake at 400°F (200°C) for the first 10 minutes. Reduce the temperature to 350°F (175°C) and continue baking for an additional 15-20 minutes.

While the puffs are baking, heat 250ml of whole milk and 2 teaspoon of instant coffee in a saucepan until it’s just about to boil. In a separate bowl, whisk together 3 large egg yolks, 40g of granulated sugar, and 20g of cornstarch until combined.

Once the coffee custard is thick, remove it from heat and let it cool for a few minutes. In a separate bowl, whip 150ml of heavy cream until soft peaks form, then gently fold it into the cooled coffee custard.

Once the choux puffs have cooled, use a sharp knife to cut a small hole in the bottom or side of each puff. Fill a piping bag with the chilled coffee filling and gently squeeze it into each puff.