Pour the cream into the jar – if you want salted butter, you can add it at this stage. The jar should have room at the top to allow the cream to move and separate.
Grab the jar and shake it steadily and continuously for 10 minutes.
Butter!
Notes
If you are using a food processor or mixer, use the pastry blade or a whisk attachment. Press "blend" or just keep it at a steady, rapid pace. The mixture will whip, then seize, then break down, separating into solid and liquid parts. When you can hear the solids thump against the sides of the jar or the bowl, this is the time to strain off the liquid.The liquid can be saved: This is buttermilk, a wonderful ingredient in pancakes, used as a marinade for poultry, or in other recipes. Buttermilk can be stored in a mason jar in your fridge until you're ready to use it in a recipe, for up to two weeks.The freshly made butter you've just strained can be mashed with a potato masher until it's all blended together, and patted into a small ramekin for serving.