While the milk is being heated, place the sugar, egg and yolks, cornstarch, cocoa powder and salt in a bowl. Whisk until you have a thick, smooth mix. Place this egg mix on top of a towel or napkin (to prevent it from slipping), and then set it aside until the milk comes to a boil.
As soon as the milk starts to boil, remove it from the heat. Slowly pour about one half of the hot milk in a thin stream, into the egg mix, WHILE WHISKING CONSTANTLY to temper the egg mix. When the eggs have been tempered, add the egg mix back into the hot milk in the saucepan.
Heat the custard base, over medium heat, while whisking vigorously until it starts to thicken - this should take about 1 - 2 minutes (depending on the heat level).
While whisking, let the custard come to a boil (the custard will release bubbles). When the first bubbles break the surface, lower the heat to low and immediately add the bittersweet chocolate chips. Whisk until the chocolate is completely melted. Remove from the heat and add the butter. Whisk in the butter, until that too is completely mixed into the custard.
Use as needed after the chocolate pastry cream has been chilled. You may need to whisk the pastry cream to make it smooth again before use.