I'm aware that many bakers use unsalted butter in their SMBC recipes but I never use it. The difference between unsalted and salted butter and just 1/4 teaspoon of salt per stick. It's not enough to make a huge difference in most baking recipes so I never bother to buy it any more, largely because I'm lazy and don't want to worry about keeping multiple types of butter in my fridge, but also because I have been burned too many times to count by accidentally spreading unsalted butter on my morning toast. I have made SMBC both ways with both types of butter (if using unsalted butter you will want to add a little extra salt anyway) and I have never had any complaints about my SMBC made with salted butter being too salty.