Preheat your oven to 425°F (220°C).

In a saucepan, combine the water, ½ cup unsalted butter, sugar, and salt. Bring to a boil over medium heat.

Once boiling, add 1 cup of flour all at once. Stir quickly until the mixture forms a ball and pulls away from the sides of the pan. Remove from heat.

Allow the dough to cool for about 5 minutes. Then, add eggs one at a time, mixing well after each addition until smooth and glossy.

Transfer the dough to a piping bag and pipe small mounds onto a baking sheet lined with parchment paper.

Bake for 20-25 minutes, or until golden brown. Let them cool completely.

In a bowl, mix 4 tbsp softened butter, ¼ cup brown sugar, ⅓ cup flour, and 1 ½ cups milk. Heat this mixture in a saucepan over medium heat, whisking constantly.

Once it starts to thicken, add 3 tbsp instant coffee and ½ cup sugar. Stir until dissolved.

Remove from heat, add ¼ cup cornstarch, and stir until mixed well. Let it cool.

Once cooled, fold in the whipped heavy cream and vanilla.

Fill the cooled choux puffs with the coffee cream mixture.