I had some cream leftover from a cake I frosted a while back, I really needed to use it up. I didn't know what to do with it though, I don't make too much with cream in it. There was a time when I was all about using real cream in my coffee but now it tastes greasy to me.
I finally decided we should make butter with it. I washed out a jar, pulled out the cream and noticed it was outdated by two weeks. I smelled it and it smelled fine, so I went with it. The cream wasn't lumpy or moldy and it tasted fine, yes I stuck my finger in it. Most butters used to be made with slightly sour milk for maximum flavor. This wasn't sour by any means.
So we poured it in to the clean canning jar and got to work shaking. I bet you could do it in food processor, easily.
At first it looked looked like whipped cream, then it was lumpy and grainy. Eventually it was a mega lump of butter and some sweet butter milk. I put it in a bowl and pressed out any milk. We served our new butter with hot baked potatoes! If you plan to keep it for a while make sure you press out as much of the milk as possible.
You can use any amount of cream you happen to have on hand and you'll get about half as much butter as cream you start with. The butter milk is sweet and good to drink, too.
Peace and Love,