I always use vinegar to wipe the rims of the jars, but canning butter is a new one on me. I make ghee and know that I can store it in the cupboard for a month or so and it is still good, but it has the dairy solids removed at that point.
A friend of mine was showing me her food storage once and she had a dozen jars of butter and I asked her about how she canned them. She just melted the butter and put it the hot jars, turned them over and let them seal. I said that I didn't think that would be safe storage since it was a dairy product that needed to be kept cold; but she had been told by a fellow church goer that it was the way to do it. A month later I got a message from her saying that I was right, all that butter went bad.
I may have to try canning butter sometime. And maybe can some ghee. It would be nice to have on hand whenever I need it.