Recipe: https://www.youtube.com/watch... (I usually abhor getting cooking recipes from Youtube -- in fact it makes me fucking ranty -- but this is the rare high-efficiency low-chatter cooking video for which both the commentary and the visuals are purely for serving the goal of making a thing.)
This started as two quarts of heavy cream plus four or five ounces of homemade yogurt, mixed together and left to curdle on the table for a couple days. It ended as about 680 grams (a pound and a half) of cultured butter plus nearly a liter of cultured buttermilk.
This is the second time I've done this and what I've learned so far is:
1: There's no point in dealing with small quantities, because a large amount is going to make just as much of a mess and take just as much effort (in fact, net result is less waste, see below). You also benefit from volume pricing if you have access to Costco or somesuch.
2: However much time you think this will take, it'll take longer. The cream should curdle to a pretty solid consistency, whipping a half-gallon of cream into butter takes about three times as long as whipping a quart of cream into butter, and pressing the water out of the butter can be a never-ending process if you let it.
2.5: Culturing the butter is absolutely necessary. Without that period of -- let's not be too vague about this, it's managed decay -- the result is bland-ass stuff and you wasted your money and time. Non-cultured salted butter tastes exactly the same as what you can get at the store (modulo your access to local good-quality dairy product, but that's a separate discussion).
3: This is messy as hell. You have to use a stand mixer to whip the cream into butter, which absolutely positively requires getting it into a state where it's flinging buttermilk all over the house; you can shield the top of the mixing bowl as thoroughly as possible and it will not be enough (on the upside, you'll spray about as much buttermilk regardless of the quantity you're making, so make a lot). And pressing the water out of the butter literally involves mashing it over and over again, eventually spreading and smearing it around all visible surfaces. Rolling it on a cutting board was my idea to expedite the process but the reality was this made the butter warm and soften faster than is ideal, so I have to think up another method.
3.5: Forgot to mention: The USDA says the standard salt content in salted butter is 1.6-1.7% by weight. Freshly salted butter is incredibly salty, so you have to let it sit in the fridge overnight at a minimum before serving.
Since this is meant to be used as table butter rather than cooking butter, reaching a target water ratio is less urgent. If you let the butter soften at room temperature (which we do, in small quantities at a time, for use on toast, biscuits and whatnot), it tends to sweat out the water on its own, which isn't isn't harmful at all, just less than perfect, in terms of presentability.
This was roughly $6-8 worth of ingredients -- thanks, Costco!. The result is considerably more expensive than buying a pound and a half of salted butter, but considerably cheaper than buying a pound and a half of high-quality imported cultured butter. Not to mention the cultured buttermilk that you get for more-or-less free. Next time I do this I want to source some local heavy cream, but that is going to raise the cost, and the stakes for success, by quite a lot. On the other hand, I can make really tasty butter and buttermilk from commodity-grade cream, so I want to see how much better local dairy cream can make it.