If you want to get ALL the nutrients from the milk, just drink it, or put it on cereal, or cook some dish that includes the milk as an ingredient. You can also make yoghurt (add a culture, let it sit at the right temperature). Yoghurt does not involve discarding any part of the milk.
When you...
Butter is mostly fat. So check the labels on a milk jug and a butter package, and pay attention to the fat.
At least for USA foods, the nutrition labels will information like how much fat per serving, and how many servings per package. Multiply those two numbers, and you know how much fat is in...