How can the pastry cream be made with chocolate

Chocolate contains fat and sugar that will throw off the ratios of a pastry cream if you just add it, so I'd recommend just using a recipe developed with chocolate, like the pastry cream here (minus the spices): https://food52.com/recipes/16168-churro-inspired-eclairs-with-spicy-chocolate-pastry-cream

You can see that compared to the vanilla one, it has more liquid and less sugar to compensate for the chocolate.