Just add some semi sweet chocolate to the vanilla pastry cream...
For a recipe calling for 4 egg yolks, I use 1/2 a bag of semi sweet dark chocolate chips....and melt them right into the mixture.
Linda C

There are several flavor variations including chocolate for Creme Patisserie below & you sure can't go wrong with any recipe from Joy of Baking.

Here is a link that might be useful: Creme Patisserie