Take a combination of sweet cream and already-cultured cream (such as cream skimmed off the top of raw yogurt) and let culture together in fridge for a week or so.**
Once cream is cultured according to one of the above methods, move to fridge to chill.
Add cultured cream to your mixer, food processor, or high-powered blender. Fill it to less than half because the cream will expand during churning. (Think whipped cream.)
Put on the lid of whichever appliance you're using, and turn on. The process of churning may take between 5 and 10 minutes.
If you're using a blender such as a Vitamix, use the tamper to keep the cream circulating and make sure to blend on setting 5 or 6 so the butter doesn't heat up.
The cream will go through several different stages on its way to cultured butter. First, it will expand and become whipped cream. Then the butter solids will start to clump together and separate from the liquid (buttermilk). The whole mixture will look grainy. Finally, the solids will continue to clump together until big chunks of butter are floating around in the buttermilk.
Churn just a minute or so more to complete the process of clumping the butter solids.
Transfer butter solids to a large bowl using a sieve to strain out the buttermilk. If you'd like, collect the buttermilk to use in baking, smoothies, for soaking grains -- you name it!
Repeat the above process until the rest of your cultured cream is processed into butter solids and collected in the bowl.
Now, add cold, clean water to bowl.
Use a hard spoon (such as a bamboo spatula) -- or if it's a large batch, use your hands -- to press and fold the butter into the sides of the bowl. The water will turn cloudy as the remaining buttermilk releases from the butter solids.
Change the water repeatedly as you wash the butter. Once the water stays clear, you have clean, cultured butter!
Discard the washing water for your animals or the compost — it isn't true buttermilk because it is too watered down.
Press and fold the butter a few more times to release any trapped water.
Mix in sea salt, to taste.
Transfer butter to a butter mold — or shape it into logs. If using a mold, transfer butter to mold until full. Scrape off the mold. Release the butter from the mold onto a sheet of natural wax or parchment paper.
Whether you've molded or just hand-shaped your butter, gently wrap it in the parchment paper as it will still be soft.
Transfer to freezer if not expecting to use right away (make sure to put wrapped butter in a ziplock bag for better storage), or store in the fridge if you're planning on using within 1 or 2 weeks. The more buttermilk you rinsed out of your butter, the longer it will last.
*Mesophilic culture (1/8 teaspoon mesophilic culture per 1/2 gallon cream), buttermilk or piima culture (1 to 2 tablespoons per 1 pint cream), or even kefir grains!
**I actually use the above method #3 (Refrigerator Culture Method). I collect cream from the top of our jugs of raw milk and even our already-cultured raw yogurt. Then I let it sit in the fridge in jars, slowly culturing, while I build up enough to make a big batch of cultured butter. I then move it to room temperature a couple hours before I start to make the butter.