Just from my experience with the high butterfat content of my cow's milk this spring, I doubt that using butter would work. Some of the butterfat ends up separating out and won't mix back into the milk, no matter how much I stir it - or how much I shake before warming the milk in the kettle. I've even ended up with some of the butterfat on the mat under the mold when the cheese is in the press. Such a waste!