Mix the starch with the sugar (if necessary, see below) and then stir in the cold milk and cream mixture, tablespoon at a time, until a thick mixture is formed.
Heat the milk and cream mixture on a medium flame and add the chocolate (it does not have to be grated, it is enough to break it into pieces). Bring to the boil, stirring constantly. Remove from the flame and stir in the starch, put back on the stove and bring to the boil again.
Then pour the pudding into a serving bowl and let cool. If you want, you can put cling film on the pudding immediately after pouring it off, then there is no skin. It is best to cool the pudding at room temperature.
A little note about sugar: I add sugar when I use dark chocolate types (70% or more). The normal chocolates have enough sugar for my taste. You can tell if it`s enough for you when you taste the chocolate milk after the chocolate has melted.