Cultured Butter

Add 1 tablespoon of mesophilic yogurt or cultured buttermilk per cup heavy cream. Mix well.
Culture 12-24 hours at 70º-77ºF


Pour butter into a small bowl.
Wash butter with filtered water, pressing out any remaining buttermilk with a spoon.
When the water runs clear, the butter should be free of buttermilk. Leaving buttermilk in the butter will cause the butter to spoil quickly.
Salt the butter and add herbs, if desired.
Wrap butter in wax paper and store in the refrigerator or freezer.

https://www.culturesforhealth.com/learn/cream/how-to-make-cultured-butter/