I’ve always had a problem measuring butter in terms of volume by cup fractions. The empty space between the chunks of butter mean there’s no way to get a good measurement, so I searched the internet for a solution. Some people suggested softening the butter so that it could be pressed down in the cup measure, however the method that appealed to me involves some very basic physics (a school subject I dropped as soon as I could because I was so bad at it!).
In the displacement method, you ‘displace’ water with the item you are trying to measure the volume of. For example, if you require 1/2 cup butter, fill the cup measure with 1/2 cup water then add butter till you reach the 1 cup level.
However, I did find today that you need to make sure all the butter is submerged in the water to get the right measurement. Which means that you need to use a large enough measuring cup to put in sufficient water. For example, today I needed 1 cup of butter but was using a 1 cup measuring jug. I used 1/2 cup water, into which I could only squeeze 1/4 cup of butter, so I had to measure out 1/4 cup butter four times over.
Read the discussion on the displacement method from Chowhound.com.
Professional bakers always measure by weight, so you might want to check out volume-weight conversions for butter. I hate measurements that aren’t round numbers so this table made me feel very lazy about weighing :P.