Put the milk in a pan and let it heat up a bit to make it easier to dissolve the chocolate powder, add the chocolate powder and dissolve it. Add the sugar and dissolve it too, add the butter and stir until it melts, after that, turn up the heat to high (leave it on high until it starts to boil, then turn it down to medium again).
Stir until thickened.
Refrigerate for 1 hour.