Salted butter
Closest match1 cup salted butter for 1 cup unsalted butter — then cut the recipe's added salt by 1/2 tspThe two are the same fat; salted butter simply carries roughly half a teaspoon of salt per cup, which you take back out elsewhere.
MethodIf the recipe contains less than 1/2 tsp of salt in total, leave it out entirely and accept a slightly saltier result rather than trying to go negative.
Trade-offSalt levels vary by brand, so precise work like caramel or shortbread can taste a little off. Everything else will be fine.