Contrary to most recipes (and bakers), I almost always use salted butter in baked goods. The reason most recipes call for unsalted butter is to limit the amount of salt in a recipe. However, I believe that most sweet treats benefit from the additional salt, and the salted butter adds such a minimal amount, it’s rarely detected anyway. But, ultimately, this is a matter of taste. Therefore, most of my recipes just say “butter”, leaving it up to the baker to decide. There are a few place where I specify unsalted butter – such as in Italian Meringue Buttercream – because it really does make the buttercream too salty. So – to answer your question: Use whatever butter you have on hand for the cake, salted or unsalted.  For the buttercream, definitely use unsalted.