Pour cream into mason jar (or other vessel) until it's 1/2 full.
Screw on the lid (if using a mason jar). Be sure the lid is leakproof.
Shake the jar until you have one solid lump of butterfat that has separated out from the buttermilk. (If using a stand mixer or electric butter churn, see directions in the post above).
Once your butter is in one solid lump, strain out the buttermilk and transfer butter to a clean glass bowl.
Wash your butter by running room temperature water (about 68 degrees) over the butter and mixing with your hands.
Change out the water 3-4 times until the water runs clear when mixing the butter.
Salt your butter (1/8 teaspoon for 2 cups cream, or see other ratios above) and mix well, straining out any additional liquid.
Spread your butter thin onto the side of your bowl and let it rest for 5 minutes to allow any excess water to drain out.
Wrap your butter in plastic wrap so it's airtight.
Store the butter in the refrigerator for 1-3 weeks, or freeze for up to a year.