In a saucepan warm the condensed milk. Add the chocolate and stir until fully dissolved. Do not boil. Allow this mixture to cool completely then put everything in a blender and zap it for about 15-20 seconds. Drink
This will keep for 4-6 weeks in a tightly sealed bottle in the fridge, though it’s unlikely to hang around that long. You may need to shake the bottle gently before use.