You can add a tablespoon of buttermilk to heavy cream, let it ferment and then make cultured butter.I love learning new things, and I’ve already learned a new thing today.
Buttermilk, like yogurt, only has an expiration date because the FDA requires it; so unless it has mold, don’t throw it away. When your jug of buttermilk is empty, refill it with regular milk and leave it on the counter a day or two until it thickens and voila, more buttermilk. It is made with a mesophilic culture, ie it will ferment on your counter. I had no idea.
It is very tasty!