Joy Wilson writes about why unsalted butter is better,

Unsalted butter ensures that you can control the amount of salt you add to your cakes, cookies and Strawberry Cream Puffs. Different companies add different amounts of salt to their butter. How are you to know how salty your butter is, and how you should adjust the salt in your recipe? It’s too much of a guessing game. Removing the salt from the butter equation puts you in charge of your cookie salting.

- Salt is a preservative. Salted butter has a longer shelf life than unsalted butter. That means that unsalted butter is typically fresher.

- Salt can mask flavors! You may not be able to taste or smell if your butter is off because clever clever salt can mask funky taste and odors. Tricky.

The solution for when you want salted butter is to make your own - with fresh ingredients of course. Joy has a great recipe for what she calls Super Herb Butter.