1. Combine cream and yogurt in a glass jar and mix well. Cover the jar and allow to sit at room temperature for 12 hours.

2. Pour the cultured cream into the bowl of an stand mixer fitted with a whisk attachment. Turn mixer on medium-high speed. Beat for 5-7 minutes, or until the cream has passed through the whipped cream stage, allowing it to separate into pale yellow butter and the off-white liquid which is the buttermilk.

3. Pour the butter and buttermilk into a fine mesh strainer set over a bowl. Use a rubber spatula to press as much of the buttermilk out as possible, then bottle it and save for another purpose. Take the fine mesh strainer with the butter in it and run cold water over it in the sink. Do this until the water runs clear.

4. Put the butter still in the strainer back over a bowl and, using your clean hands or a wooden spoon or spatula, knead/press it to remove the rest off the buttermilk/water. Keep kneading it until it is as "dry" as you can get it. You can add a pinch of sea salt to the butter, if you like, while you are kneading it.

4. When you have pressed all of the liquid out of your butter, put it in a covered container in the refrigerator or freezer for storage. Or, if you have a butter crock, you can store it at room temperature.