Yes you can safely use butter in your copper chef pan if you want too to cook the food and add flavor.
However when cooking in a copper chef pan there's no need to use butter or cooking oil to prevent the food from sticking.
The copper chef pan has a good non stick surface but butter in the copper chef pan will not harm the copper pan if you want to use it.
I use butter in some of my dishes when cooking in my copper chef pan and the butter has done no harm to the pan.
I like to add some butter to things such as french toast which I make using my copper chef pan and the pan has never been harmed.