Note: The quality of the cream you use will directly effect the quality (flavor, density, color) of your butter. "Good" cream is cream from cows that produce cream with a high fat content, including Jersey, Guernsey, and Brown Swiss cows. Cows that are pasture raised (grass fed) also tend to produce more flavorful cream. Quality cream also tends to look "cream colored" or yellowish.
Pour cream into a mason jar and seal the lid.
Shake for 5 - 10 minutes until butter fat (butter) and butter milk separate.
The cream will turn to whipped cream before it separates. At this point, it may feel as if nothing is happening. Keep shaking.
All of a sudden, the butter fat and butter milk will separate. You'll know when you can see a solid butter ball among liquid milk.
Pour off the butter milk. (Save to use as regular milk. It's especially good in pancakes!)
Shake once more for a 30sec - 1min to separate any remaining butter milk. Pour off milk.
Remove butter from the jar. (Here you can rinse the butter under cold water to remove any remaining butter milk. I don't often find this necessary, especially if you're using really rich cream.)
Wrap butter in wax paper or place in a sealed container. Refrigerate for an hour plus to firm up.
Serve/use cold or at room temperature as you would any other butter.
Note: Homemade butter can go rancid faster than store-bought butter, so eat up! It will taste sour if it has gone off.