Discard the vanilla pod from the hot milk. Pour the milk slowly on to the yolk mixture, stirring constantly. Pour the resulting custard into a clean pan over a medium-low heat. Heat it, stirring, until it coats the back of a wooden spoon (it needs to be quite thick for trifle, but will thicken as it cools). Pour into a jug, place a layer of cling film directly on the surface and allow to cool.