But there was something else about European butter. It was richer in flavor, with notes of hazelnuts and a pronounced and often complex tang.

After researching traditional dairy techniques in a handful of out-of-print books, Ms. St. Clair realized that what made high-end European butter taste so much better than “the bland sticks of fat in the supermarket,” she said, was that the European cream was cultured, or fermented, before it was churned.

Butter-tasting (so may varieties!) video here: 

http://www.nytimes.com/2013/10/09/dining/making-cultured-butter-at-home.html?smid=tw-nytimes&_r=0