Why choose unsalted butter
Butter is available both with and without salt. The salt is added for extra flavor and to help preserve it so it has a longer shelf life. The problem is that sometimes the salt in butter can be more than a recipe needs.
Choosing unsalted butter gives you more control over how much salt your dish contains.
If you only have salted butter, the best thing to do is omit approximately ¼ teaspoon of salt per ½ cup (one stick) of butter used in the recipe.