The butter is made by a process called "inversion," which involves making a water-in-oil emulsion (butter) from an oil-in-water emulsion (cream). Here is an infographic we made up to help understand the process (click on it for a larger view). For a much more detailed discussion, see here, here, here, here, and here.
There are lots of ways to bring about this cream inversion, but most DIY instructions seem to focus on using an electric mixer (countertop, handheld, or stick style), or the old-fashioned jar-shaking method (no need for a Shakeweight!). A few sources mention using a manual egg beater, but we couldn't find anyone who seems to have actually done it that way. Since it's super easy to make whipped cream with an egg beater, we set out to show that butter wouldn't be too much extra work. But as it turns out, we ended up learning a lesson in modern handheld appliance construction and ergonomics. Fortunately, we still got butter in the end without using anything electronic!
|After about five minutes of beating, we're almost at whipped cream stage. Lots of air has been incorporated into the oil-in-water emulsion, making it harder for the butterfat particles to move out of the way of your beater. If you want something to put on top of your 'Hog Nog, beat just a little more, then stop.|
|Take a break to drink the buttermilk and eat a cookie right away before anyone sees your liquid white gold-in-a-glass. If Katie walks into the kitchen before you finish it, reluctantly offer her some because she made the cookie.|
|Keep working the butter until you can't get any more buttermilk to come out. You might want to switch to your hands, since they're a little better squeezing tool than a spoon. Once you can't squeeze any more out, rinse the butter under a trickle of very cold water (keep kneading it with your hands) until the liquid you squeeze out runs clear. That helps to wash out any of the water-soluble compounds (like butyric acid) that make butter taste funky after a few days.|