Culture the cream (optional) Add the cream to a bowl and whisk in two tablespoons of plain yogurt. Cover the bowl with a kitchen towel and set in a slightly warm area to culture. Allow the cream to culture for 8-12 hours or until the cream has thickened a bit and is a little foamy. It will also smell a little sour when cultured. Place the bowl in the refrigerator for up to an hour to chill before churning. If you did not culture the cream, allow it to sit out of the container for 30 minutes to warm.
Add the heavy cream to the bowl of a stand mixer and turn the mixer on to medium-high.
Churn the heavy cream until the butter seperates from the buttermilk, about 3-6 minutes.
Pour the buttermilk through a cheesecloth lined stainer. Pour the buttermilk in a container and refridgerate. Gather the butter in the cheesecloth and squeeze hard with your fists. Do this serveral times to get as much of the buttermilk out.
Hold the butter in the cheesecloth under cold water, squeeze and knead the butter to release anymore buttermilk. Do this untill the water runs clear, 3-5 miuntes.
Add the salt and knead into the butter.
Scoop the butter into a jar or roll into a log using parchment paper and store in the refridgerator. The butter will keep for 3 weeks in the refrigerator and 3 months in the freezer.