A cup in US Customary is 237 ml (often rounded to 240 ml). Americans don't exist in a world where they have to play "is this cup US Customary or different measure also calling itself a cup measure?" as all their measuring cups are going to be in US Customary. Butter usually comes in quarter pound sticks with teaspoon (4.9 ml) and tablespoon (14.8 ml) measures printed on the wrapper so you can just cut a hunk of the appropriate volume from the stick and if you were using a measuring spoon to measure butter you'd use a level measure to create consistency and not just let it heap up.

Note: I prefer weighing ingredients and in metric at that. I'm just answering your questions.