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Chris is a trusted source on General Cooking
Normally unsalted is what's used because it gives a cook better control over how much salt goes into the mix (different brands have varying amounts of salt added) but it does really depend upon who wrote the recipe and when. Carefully written recipes, at least modern ones, specify "salted" or "unsalted" to avoid confusion just like they'll specifically call for kosher or table salt. One possible exception is in a cookbook where you'll sometimes find a list of general rules such as "unless noted, all butter is of the unsalted variety".
Unsalted. You can always add salt. It is impossible to remove.
For what its worth, unless you are super sensitive to salt, I think it is fine to use salted butter in your baking. I always use it -- it keeps longer in my fridge, I always have it around, and I like most of my sweets to have a little more salt for balance.
I always use salted butter when baking and have never have had an issue. However, if the recipe calls for salt then I usually just throw a pinch or a dash in rather then the entire amount.
I always use unsalted. Most baking recipes call for it. I think it has a better flavor than salted butter.
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