Q: Lots of recipes call for unsalted butter, which I don't usually have on hand. Do I need to use unsalted butter? How would I replace it with salted butter?-- Michelle
Some cooks claim that there is a noticeable taste difference between salted and unsalted butter, so my first suggestion would be to try it for yourself. Make the same recipe with salted and unsalted butter and see if you can tell, or if you care about, the difference.
Generally, you can replace unsalted butter with salted butter measure for measure. If you do, then you will need to reduce the salt in the overall recipe by about 1/2 teaspoon per cup of butter used. For some recipes, this may not matter. For others, like classic puff pastry, you may find it too salty if you don't.
There is some variation in the manufacture of butter. Some brands may contain more or less salt, so you may want to start with less added salt and taste test, where possible. Don't test anything containing raw eggs or other ingredients that may not be safe until they are cooked.
To see how I came to the amount of 1/2 teaspoon per cup, note that the USDA National Nutrient Database for Standard Reference shows one cup of "Butter, salted" as containing 1308 mg of sodium and 1 cup of "Butter, without salt" as containing 25 mg, for a difference of 1283 mg. They also show table salt as containing 2325 mg of sodium. Dividing 1283 mg of sodium by 2325 mg sodium per teaspoon of salt gives 0.55 or just over 1/2 teaspoon.
Also, read the related KitchenSavvy article More on Unsalted Butter about why recipes actually call for unsalted butter.