Heat 200ml's of the milk and coffee in a pot. Mix the custard powder, sugar, and cocoa together with 50ml's of milk in a separate glass. Add to the hot coffee milk in the pot and whisk until very thick. Pour immediately into ramekins and smooth the top. Allow to cool for 10 minutes. Refrigerate for 2 hours before serving.
You can fancy this up by serving it with crushed chocolate on the top, or with cigar biscuits and a touch of edible glitter on the top. This is way easier than making proper chocolate mousse, and the taste is just as good as pot au choc.