Cookbook authors and celebrity chefs almost invariably call for unsalted butter in their recipes, because it's a more consistent product than the salted variety. The level of salt added to salted butter varies regionally and by brand, making it an unknown quantity in finely tuned dishes. If salted butter is all you normally keep on hand, that doesn't mean you should abandon the recipe if it calls for it. At worst, you'll need to make some minor adjustments.
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Keeping Your Balance
The salt in butter is partly a preservative, as it is in other foods, but manufacturers add it in greater or lesser quantities to conform to local tastes. On the average, one stick of butter -- a quarter-pound, or half-cup -- contains 1/4 to 3/8 teaspoon of salt. If you need to substitute salted butter for unsalted, simply reduce the recipe's remaining salt by the corresponding amount. If the recipe contains other salty ingredients, such as Parmesan cheese, you might wish to start with a slightly smaller quantity and add more if needed.
Form and Function
Some recipes will still taste fine if you simply substitute salted butter for unsalted, but flavor isn't your only concern. Salt plays a part in the chemistry of many foods, and meddling with its proportions can have unexpected results. For example, the salt in yeast dough acts as a check on the yeasts, preventing them from reproducing too freely and giving your dough a sour, fermented flavor.