In a Jar Pour one cup of cream into each of two pint jars. Each jar will be half full. Then . . . shake. This will take 10-30 minutes depending on the diligence of your shakers and how cold your cream was to start with (room temperature cream will split more quickly). First the cream will be liquid, then it will fluff up into whipped cream, then it will split into yellow solids and a clear liquid. This is butter solids and buttermilk!
In a Mixer Pour the cream into the bowl of a stand mixer with the wire whisk attachment. You might need to begin mixing at a low speed to avoid splattering. Increase the speed to full when possible. Scrape the sides of the bowl towards the end of the process. It will go through the stages of cream, to whipped cream, to butter solids and buttermilk in about 10 minutes, longer if your cream was very cold to start with. When it’s done you’ll have yellow butter crumbles in cloudy liquid buttermilk. (If you’re not using it for scone-baking, keep mixing a bit longer until it’s smooth and congealed onto the whisk.)
Finish: Pour the buttermilk off. I like to dump the whole thing into a wire sieve over a large measuring jar and let it drain, but you can also pour it off the side.
If you’re making the scones, gently scoop half of the crumbly butter solids out and gently spread them in a thin layer on a piece of wax or parchment paper on a plate or small baking sheet. Avoid squishing them together. (A kitchen scale is ideal for this. You want 4-5 ounces of butter.) If your butter has congealed, just spread it into a layer. Place the butter in the freezer while you begin to prepare the scones.
If you’re not making the scones, you’re just about done! The buttermilk can be reserved in the fridge for another use. The butter can be served right away as is, just press it a bit with a rubber spatula to get as much liquid out as you can, use your hands to form it if necessary, then scoop it into a bowl. You can add salt, or herbs, or honey as desired. It can be shaped in a mold for added extra-ness. If you don’t plan to finish the butter the same day, it should be rinsed well in cold water to remove any excess buttermilk (which can eventually go rancid). Rinsed butter will stay good for 3-5 days at room temperature, and about 7-10 days refrigerated.