If the recipe indicates unsalted butter, it is always best to use it. Unsalted butter is often specified for delicate pastries for fruit tarts and tartlets, and for Danish pastries and croissants. It is also used for icings, cakes, tea breads, sweet biscuits, baked desserts such as crumbles, confectionery like butterscotch, and other sweet foods, where even a pinch of salt would be obvious and would change the flavor of the dish. It is also better to use unsalted butter for delicate sauces such as hollandaise, and then to add salt to taste to the sauce.
Sometimes, however, salted butter is a crucial part of the dish because the salt highlights the flavors of the other ingredients.
** Asian Recipes