Cultured dairy has been fermented. When making cultured butter, it's an extra step before churning. The cream needs to become acidic and this is achieved through the addition of cultures.
They can be bought from the store (not ideal), found in the good quality milk you buy, in kefir grains or even in your own culture you make, known as clabber.
Depending on which fermentation method you prefer you can make an array of dairy products.
Using kefir as a starter culture is the easiest to start with and will give the most reliable and consistent outcome.
Kefir grains are fed a daily dose of milk and overnight this ferments and becomes a delicious drink.
This liquid can be used to make yoghurt or added to fresh cream to make cultured butter. It can even be used in the production of cheese. Through the fermentation the bacteria convert the lactose ( sugar found in milk ) to lactic acid creating an acidic environment and thus a thicker cream, known commonly as crème fraîche. This extra step of culturing adds not only, extended shelf life but also a wide array of beneficial probiotics to your diet.