I had a ton of leftover whipped cream, and someone recommended continuing to whip it until it made butter, which freezes better than whipped cream. (I'm going out of town and can't eat it all.) The thing is, I forgot until I had finished rinsing the butter that I had added sugar and bourbon vanilla extract to the whipped cream for flavor, so now I have super-sweet, slightly alcoholic fresh butter.
So if anyone was thinking about using up extra whipped cream that way, bear that in mind... Alternatively, if you want custom-flavored butter, you could do it on purpose. :)