European butter has more fat.
Commercial butter is 80 to 82 percent milk fat (also called butterfat), 16 to 17 percent water, and 1 to 2 percent milk solids (plus about 2 percent salt if salted).
The United States Department of Agriculture (USDA) sets the lower limit of butterfat content for American butter at 80 percent, while most European butters contain a minimum of 82 or even as much as 84 percent.
That may not sound like much of a difference, but more fat means less water and hence a richer, creamier product. Pastry chefs often refer to European butter as “dry butter.” Moreover, higher-fat butter makes smoother sauces and flakier and more flavorful pastries. (Compare the croissants you had in France with those anything-that’s-bent-in-the-middle American imitations.)
Butter, as you know, is made by churning cream or whole, unhomogenized milk. The agitation of churning breaks up the emulsion (tiny globules of fat suspended in water) in the cream, so that the fat globules are free to coalesce into granules the size of rice grains.
These then mat together and separate from the watery part of the milk, called the buttermilk. (Today’s cultured buttermilk products have been processed further.) The fat is then washed with water and “worked” to squeeze out more buttermilk. European butter is generally made in small batches, allowing for more complete removal of the buttermilk.
Some American brands of European-style butter are Keller’s, formerly known as Plugrá, a pun on the French plus gras, meaning “more fat” Land O’ Lakes Ultra Creamy; and Challenge. European butters imported from France and Denmark are available in specialty stores.
Bring lots of euros.