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About five years ago, my brother-in-law shared with us the tradition of making butter by hand to serve at Thanksgiving dinner. We each got our little jar of heavy cream with a bit of salt added, and sat shaking the darn things for 1-2 hours while we watched a movie. I like the final product well enough, but wanted to find a better way.
After some successful but labor-intensive experiments involving a backpack and a treadmill, I hit on the perfect solution: repurposing the rock tumbler as a butter churn.
You will need: a rock tumbler, a container that fits on it relatively well, and a small washable clean something (we used a small silicone condiment bowl) to use as a paddle. Also heavy cream, a bit of salt, and any other flavorings you'd like to add. (Ideas: truffle oil, herbs, honey...) Take care not to buy sweetened cream!
The container shown here is from Gatorade powder.
Put your cream (with some room to slosh) and your butter paddle into the container, seal it tight, and put it on the rock tumbler.
Keep checking the container, and soon you will have butter! (You will have to dig out your butter paddle.) Enjoy.