Cultured butters are a lot like sweet cream butters, save for the fact that they’re made with cream that’s been allowed to ferment slightly. Lactic acid go to work on the sugars in the cream, digesting them and creating a variety of by-products including acetic acid, acetoin, ethyl formate, ethyl acetate, 2-butanone and especially diacetyl. The effect of those by-products is to heighten the flavor of the butter, in much the same way a starter heightens the flavor of bread.
Once upon a time this action happened as a matter of course. Milk left our in pails for pickup by the local dairy would sour naturally…sometimes a lot. As a result, flavor could vary from mildly interesting to extremely strong and cheesy, especially when the weather was warm. To gain control over the process, dairies on the Continent began growing and keeping milder cultures on-premises, then inoculating their sweet cream butters with it once they had been churned. Over time the culture would spread through the entire mass. This best-of-both-worlds approach (the freshness of sweet cream plus the flavor of a culture) is how butter is made in much of Western Europe to this day.
A bacterial culture, however, is not the only difference between European cultured and mass-market American sweet cream butters. Fat content in Europeans butters is usually higher, about 2%, which doesn’t sound like a lot but does make a difference in pastry making, especially in laminated doughs. Indeed there are a variety of specialty butters made in Europe including beurre sec (dry butter), beurre pâtissier (pastry butter) and beurre concentré (concentrated butter) that are even higher in butterfat…up to 99%.
These days there are some very good cultured butters made in North America, some of them with a butterfat content that’s higher than standard European butter. However it’s important to note that fat content is by no means the be-all and end-all of butter. Consistency is every bit as important, and that comes from careful handling of the cream and precise attention to the details of the butter making process.