If your dairy has refrigeration and your people have invented pasteurization, your butter has no culture. If not, while you let your milk sit overnight to let the cream rise to the top, all sorts of bacteria are working away on that delicious, nutritious milk. Depending on what bacteria choose to colonize, you can get a variety of results, including the death of your family. But if all goes well, what you skim off the top in the morning will be cultured cream, or creme fraiche. Look at those tasty fermentation bubbles!
Churn or whip that creme fraiche, and you get cultured butter. Here is a refresher course on how that works. Depending on the bacteria involved, this butter will taste anywhere from very slightly tangy to very tangy, and you can now sell it at a premium to foodies, hipsters, and health food nuts. The resulting buttermilk will be actual, real buttermilk. The buttermilk you buy at the store is not a product of butter making anymore, it is a cultured product like yogurt.
But what if you, like me, do not have a home dairy? What if your enthusiasm for history and your hands-on experiences milking cows have left you totally unwilling to consume raw milk? Fear not. You can cheat. Set up a date between your cream and the bacteria of your choice. I used milk kefir starter. You can also use buttermilk from the store, like these nice people. Let sit overnight, and you now have sour cream!
Mmmmmm. Butter. Now let us conclude by mocking this commercial claiming that butter is evil and that butter substitute will save us all.