Pour the milk into a medium size saucepan and add the instant coffee. Then heat the milk by bringing it to a boil over medium heat.
In a big mixing bowl, combine the egg, sugar, cornstarch, cocoa powder and whisk until combined.
Slowly whisk in the hot milk. Strain the mixture back into the saucepan. Place the pot over medium heat and cook while whisking continuously until thickened.
Remove the saucepan from the heat and add the soften butter. Mix until all combined well. Transfer the pastry cream into a clean bowl and cover with plastic wrap, lightly pressing the plastic against the surface to prevent crust from forming.
Let it cool to room temperature, and then chill in the refrigerator for at least 2 hours (or overnight). Whip the custard for 1 minute before using.
Notes
To make filling for cream puffs or éclairs, just whip 80-100 ml cold heavy cream to medium peaks (be careful not to overbeat the cream) and add to the custard. Transfer into a pastry bag and use it for your favorite desserts.You can make the custard ahead of time, it will keep for a couple of days in an airtight container in the fridge, then give it a quick whisk to break it up as it will set and loose that silky texture. However, I would not add the whipped cream until you were ready to serve it. It is best served immediately.