Place unopened cream on the counter and leave at room temperature for about 8 hours to sour a bit.
Pour cream into food processor, using a knife to scrape out thick cream that sticks to the inside of the bottle.
Process until cream separates into curds (butter globules) and whey (buttermilk). About 1 minute.
Tip into a strainer set over a suitable metal or glass bowl. Let sit for a minute or two to drain. Do not press butter as you don't want to push it through the seive.
Tip butter into a separate metal or wooden bowl. Stir and press it with a wooden spoon, causing small amounts of buttermilk to run out - tip the buttermilk into the container with the rest of it. Do this over and over until you've got as much buttermilk out as you can. Pour buttermilk into clean glass jar, cap and refridgerate.
Now, slowly run cold water over the butter, holding your container on an angle over the sink and working and pressing the butter over and over so that the water washes out all remaining buttermilk residue. (This milky water is not kept, unless you want to use it in the garden). It is important to get all traces of buttermilk out - any left behind will cause the butter to go rancid much more quickly than it should.
Form clean butter into whatever shape you desire and place in container. Refigerate, or freeze for longer storage.
The buttermilk can be used in baking, or as a starter for lacto-fermentation of vegetables. Using whey/buttermilk in fermation means that less salt can be used, as the whey speeds up the process of formation of sufficient lacto-acid to prevent the vegetables from spoiling.
I had an unopened bottle of cream I bought for Christmas but didn't use. It's 4 days past it's best-before date, but perfectly fine. Since my husband and I are mostly dairy free, what to do with it? I decided to make cultured butter and buttermilk - very easy!
500ml of cream yielded 272g butter and 3/4 cup buttermilk