To clean any grease stain, be it cooking oil, butter or even automotive grease, I use automotive degreaser. They use it to remove grease from cement in autobody shops. You can get it at Walmart.
Just spray it on, let it sit for a minute and wash as usual. I have never had it ruin any clothing, even expensive blouses. It gets it out every time even if it's already been through the dryer. Of course, you should test an inconspicuous spot first though.
