I am talking especially about Cadbury Bournvita.
That powder has tiny and somewhat hard chunks in it which don't get dissolved easily. How do dissolve it completely in milk?
I think the Q&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-)
As Ecnerwal answer implies, there are various ways, but from my experience with baking-grade cocoa: go with pasting. Add a LITTLE milk at first, stir, repeat until you have a paste, continue adding milk slowly and stirring until you have a liquid. Then add all the milk and/or other liquids you want.
I do this for making even hot chocolate drinks, because it dissolves almost-undissolvable baking-cocoa just fine :).
An undersized wire loop wisk is indispensable for mixing dry products that clump into liquids smoothly, from powdered milk to dry gravy mixes to protein powders.