Pour the cream into a pint-sized (2 cup) mason jar or other pint-sized jar with a tight-fitting lid. Put on the lid and make sure it is nice and tight.
Shake the jar vigorously for several minutes. After about 4 to 5 minutes, you will notice that there doesn't seem to be any liquid sloshing around any longer. At this point, you have whipped cream.
Continue shaking vigorously for several more minutes. For a few minutes, it's going to seem like nothing is happening, and that's totally normal. Just keep shaking.
At about the 9 to 10-minute mark, you will begin to notice a thin liquid starting to slosh around in the jar. Keep shaking. In another minute or so, you will see and feel a ball of yellow butter bouncing around in the jar with that thin liquid. You're done shaking!
Rinse the butter with cold water as you knead it for a few minutes to remove any excess buttermilk. Note: This step is important, especially If you won't be using all of your butter immediately, as any excess buttermilk can cause your butter to spoil faster. The more buttermilk you remove, the longer your butter will last.
Gently pat the butter dry with a paper towel. Place the butter in a bowl and add salt to taste (I use ¼ teaspoon of sea salt, but you may want to start with less); stir until well-combined. Adding salt is optional, but it adds a lot of flavor to the butter, which can be bland without it.
If you are not using your butter immediately, place it in an air-tight container and keep it in the refrigerator. Use it within a few days for optimal freshness.