Pour the cream into a large mixing bowl (or the bowl of a stand mixer)
Beat the cream at a medium/high speed. Cover the bowl with a kitchen towel if it's spattering out of the bowl.
Continue mixing until the cream thickens and starts to separate into chunks of butter and buttermilk. This takes quite a bit of mixing before you get to this point.
Drain the buttermilk off of the butter, using a fine mesh strainer or a piece of cheesecloth draped over a bowl. Save and refrigerate the buttermilk for use in recipes.
Cover the butter in the bowl with a bit of cold water. Then mix it again with your mixer. Pour off the cloudy water, and add fresh water to the bowl. Mix again, and continue this until the water remains clear. Pour off any excess water.
Add the salt, if using. We use about 1/2 teaspoon of salt. Mix the salt into the butter with your mixer.
Drain any remaining excess water off the butter, using a rubber spatula or cheesecloth to squeeze out the water.
Store your completed butter in the refrigerator for a week or two.