How do I dissolve chunky powder completely in milk?
I think the Q&amp;A linked by @Joe has most of the tricks in it. Hot, paste, make syrup, blender, etc. Mixing stuff into cold milk (unless specially prepped for that) is not a good scene. Surprising they haven't done better at that given the marketing, but corporate competence is a rare thing - they may be too big to get someone that knows how to make a powder dissolve in cold milk working where they need to work - Nestle solved that one (at least) 50 years ago. Too many vice presidents, and not enough food science engineers? Puzzling.

If you have "small hard chunks" either pre-grind dry in a mortar and pestle, or grind the paste step suggested in the other answer in a mortar and pestle with a little liquid.

Or contact Cadbury all wide eyed and innocent and ask why you get little hard chunks when you mix their product (I assume, as instructed on the package) 8-)