Not sure this will help as I have no specifics, but I worked at a Starbucks and we made the base mixture for the drink from a powdered mix mixed with skim milk. The drink itself is made with a combination of the base mixture and ice and put through the blender. Ratio of ice to mix depends on the size (tall grande or venti) and is measured by the green lines on the drink cups.
As for the powdered mix, your guess is as good as mine as to what's in it!!