Hi There,
milk powder is already "cooked".. is made either by "rolling" milk into a big heated cylinder and then scraped off, or by spraying milk into a hot chamber to vaporize the water.
The differences between milk powders are generally : Fat content (use a 20%), what kind of cows, season, area of growth (fed with green grass, hay or whatever) ,production system.
What system of making chocolate you do use, what temperature, how do you grind/insert the milk powder to the chocolate and mailny what quality of milk you are using will affect the final taste of your product.
Do not compare to swiss milk chocolate (unless you live there), their cows make a very mild milk, and they add malt to the milk powder....
If you cook the milk powder in cocoa butter you will end up with caramel...... then we are in a different game...