Combine the cream and culture. Pour the cream into a very clean glass jar. Stir in the cultivated buttermilk or yogurt until evenly incorporated.

Culture for 36 hours. Cover the jar with a clean lid and leave it at a moderate room temperature, ideally around 68–72°F (20–22°C), for 36 hours. The cream should become noticeably thicker and develop a pleasantly tangy, cultured aroma.

Chill for 4 hours. Transfer the cultured cream to the refrigerator and chill for 4 hours. This firms the butterfat and makes the butter easier to churn.

Churn. Pour the chilled cultured cream into the bowl of a stand mixer fitted with the whisk attachment. Beat on medium-high. It will first become whipped cream, then look grainy, and finally separate into yellow butter solids and liquid buttermilk.

Drain. Pour off the buttermilk and save it for pancakes, biscuits, breads, or other recipes.

Wash the butter. Add very cold water to the butter and gently press or knead it with a spatula or clean hands. Pour off the cloudy water and repeat with fresh cold water until the water runs nearly clear. This removes residual buttermilk and helps the butter keep longer.

Finish. Press out as much remaining liquid as possible. Knead in the sea salt, if using. Shape the butter into a block or place it in a butter crock.