make the same as a sweet white sauce using coco powder for the chocolate taste mix cornflower, into a smooth paste with a little sugar. boil some milk add a little milk from the boiled milk into the paste, keep doing this then tip the whole lot back into the milk. and re boil normally you would add vanilla essence to taste, but in this case just add coco powder
I would try using half cocoa powder to half cornflour,then heat milk to boiling point just like you were making custard, i always add the sugar when removed from heat prevents it burning,,maybe throw in a couple of cubes of chocolate,or you could melt chocolate over a pan of simmering water add a knob of butter when melted makes it super shiny,then remove from heat add double cream till reaches consistency you like!!!