Divide the ingredients between two 1/2-cup jars with tight-fitting lids, then place lids on tightly.
Shake the jars up, down and sideways until the cream thickens and begins to come together. The butter will first start to look like whipped cream, then it will eventually begin to gather into a ball. When the cream has separated completely into a ball, floating in relatively clear liquid (the “buttermilk”), it is done.
Pour off the liquid and discard, or it may be refrigerated and saved to drink or used in another recipe.
Next, pour cold, clean water into the jar with the butter and continue to rinse until it runs mostly clear. This rinsing will help prevent your butter from turning sour.
Using a fork, mash the butter against the sides of the jar to release and remove any remaining excess water.
Cover and chill or freeze.
(Alternatively, you can place all the ingredients in the bowl of an electric mixer with a whisk attachment and whip for approximately 10 minutes, working up to high speed. Once the buttermilk separates from the butter as above, it is done.)