Adapted from The Animal Farm Buttermilk Cookbook.When it comes to making buttermilk and butter, forget your visions of slow, laborious churning. For small quantities, all it takes is a food processor and a short amount of active work.
Place cream in a medium saucepan over medium heat and very gently heat it to 70 degrees, using a thermometer to monitor it and stirring frequently with a rubber spatula to prevent scorching. (This will take only a few minutes.) Pour cream into a large bowl and stir in buttermilk. Cover bowl tightly with plastic wrap and leave at room temperature at least overnight and up to 24 hours. Try to maintain temperature at about 70 degrees. (If you have a yogurt maker, that will work beautifully.) If your kitchen is very warm, move the bowl to someplace cooler in your house, but don't put it in the refrigerator, or the culture will not be able to do its work.
The next day, remove plastic wrap and stir. The mixture should be noticeably thick. At this point, it is actually creme fraiche; scoop some out and use it as such, if you like.
Place the cultured cream in a food processor and process until it begins to form grains of butter, 3 to 4 minutes. (If butter does not form, the cultured cream may not be the right temperature. To get it to within a degree or two of 68 degrees, refrigerate it for a while if it's too warm, or let it sit in a warm place if it's too cool. Then try again.) Pour mixture through a fine sieve set over a bowl and set aside buttermilk.
Put butter (the solids in sieve) on a clean, cool work surface and gently gather it into a mass. Rinse under cold running water. Knead, folding it in thirds like an envelope, and rinse again. Repeat folding and rinsing until you have pressed out as much liquid as you can. I prefer unsalted butter, which lets you taste all the glories of good cream and the tangy lactic-acid culture you have introduced. For salted butter, though, knead in a very small amount of salt.
Transfer buttermilk to an airtight container and refrigerate. Pack butter tightly into a container with a tight-fitting lid and refrigerate.
Buttermilk and butter can be stored in refrigerator about 2 weeks.