I’m a broken record on this but I’ll say it again: any time you’re gearing up for a butter-intensive project, pay up for some of the good Euro stuff. It’ll be double the price of your garden-variety unsalted, but the taste — especially in something like a brioche where there are no other competing flavors — will be out of this world. Fresher, tangier and lighter (even though Euro-butter actually contains more fat).
I usually go for this stuff: Lurpak. The name alone kills me, but the flavor is unbelievable. One taste and you’ll be casting your eyes madly about the kitchen looking for substances to slather it on — bread, crackers, dry wall, shingles, ANYTHING!!!
Slightly cheaper is the domestically-made Plugra, an outstanding Euro-style butter, though it has a slightly less refined flavor than the real thing in my opinion. But unless Warren Buffet has recently designated you his successor and heir, don’t go hog wild with French Buerre D’Isigny. That’ll run you about twice the price of the expensive stuff. I mean a little indulgence is one thing, but let’s not go crazy.