Pour the cream into the bowl of a stand mixer, and stir it with the paddle attachment on medium-low speed. Churn the cream until it separates into butterfat and buttermilk, scraping down the sides of the bowl once about halfway through. The time will vary depending on the temperature (colder cream will turn more quickly), but it will take approximately 15 minutes, and will sound like water sloshing when it is done.
Once the buttermilk has separated from the butterfat, place the chunks of solidified butterfat into a colander and rinse it thoroughly with very cold water, squeezing it gently with your hands to release any excess buttermilk. The less buttermilk it has in it, the longer it will store.
Separate the butterfat into desired portions. Add salt if you wish and knead it into butter. You can also create a compound butter by mixing in minced garlic and chopped herbs or cinnamon and honey or any other flavorings that you like.
Use a piece of parchment paper to roll the butter into a log, shape it into blocks with your hands or press it into a ramekin with a spoon. Once you have formed it into your desired shape, carve a decoration into the top with the tip of a paring knife. To help the butter last longer, you can also wrap it tightly in several layers of plastic and store it in the freezer for up to 3 months or in the refrigerator for up to 2 weeks.