Season to taste with salt if you like.
Fit food processor with plastic blade, whisk, or normal chopping blade. Fill food processor about 1/4 - 1/2 full. Blend. The cream will go through the following stages: Sloshy, frothy, soft whipped cream, firm whipped cream, coarse whipped cream. Then, suddenly, the cream will seize, its smooth shape will collapse, and the whirring will change to sloshing. The butter is now fine grained bits of butter in buttermilk, and a few seconds later, a glob of yellowish butter will separate from milky buttermilk. Drain the buttermilk. (Buttermilk good for up to 1 week)
If you are into antiquing you can pick up a table butter churn which is like a mason jar with a small wooden paddle in it.