in a non stick saucepan add the margarine and let melt over low heat
add potato starch, cacao powder, sugar and stir all the ingredients together, until they form a paste
add the milk and stir vigorously with a whisk, until the paste is completely dissolved
add vanilla extract, keep stirring over medium heat, until the mixture thickens and reaches a dense consistency
the cream will get still a bit denser once removed from the stove
small clumps may form, I usually don't mind it, but if you want a perfectly silky chocolate pastry cream you can sift it through a fine mesh strainer using a spatula
if you are not using it immediately, place the pastry cream in a bowl and cover it with kitchen cling film touching directly the surface of the cream (this will prevent a film forming at the top of the cream)