While most Americans consume sweet cream butter, cultured butter has been around for centuries and is still the spread of choice in much of Europe. Not only does it have a lot more flavor than store bought butter, it has a longer shelf life at room temperature as well. Lucky for us, it is also very easy to make.
The traditional method involves leaving fresh cream out to sour a bit before churning it. Unfortunately almost all commercial heavy cream in the United States is sold ultra-pasteurized, a process that kills all of the natural cultures found in the cream. As a result, we have to find a way to add it back in.