Elaine Khosrova, author of Butter: A Rich History (2016), says Breton butter, like a fine wine, exhibits strong “terroir.” “Great soil, good water, clean air, the right animals, beautiful salt—Brittany just has the perfect combination,” she says. Thanks to the region’s coastal climate and rare breeds of cattle, Breton butter is golden and sweet, with a long, creamy finish. It is also distinctively saline. Salt became part of Brittany’s butter-making tradition because the area boasts abundant deposits of natural sea salt beds; plus, the region was never subject to France’s infamous harsh salt tax. Today, Breton butter is sold in three styles: doux (unsalted), demi-sel (containing 0.5–3 percent salt), and sel (containing 3 percent or more salt, about double the salt content of American supermarket salted butter).…