Should I use salted or unsalted butter if the recipe doesn’t specify

Generally, you should use unsalted butter. You can always add salt to your unsalted butter, but you can't take it out if you want it less salty!

If it's just being melted on some vegetables, then salted butter is probably fine. However, different brands of salted butter have different amounts of salt added, which makes it difficult to know how much total salt is going into your food. This is more problematic in baking. It's possible to easily oversalt or undersalt just by using a different brand — leading to unpredictable results. By using unsalted butter, the only salt remaining in the recipe is what you have added, and you have more careful control over the outcome.

If you need to substitute one for another, you can estimate how much salt is in salted butter and adjust your recipe accordingly.

If you are not sure whether a particular recipe calls for salted or unsalted, look for clues.

• Is there additional salt in the recipe? (If not, it may expect some salt from the butter. If so, it may expect unsalted butter!)
• How old is the recipe? (Newer recipes tend to assume unsalted butter. Older recipes tend to assume salted butter.)
• Where was the recipe written? Different cultures assume different butters (and salt levels may vary between countries for salted butter!); I will not give an exhaustive list here, mostly because I don't know, but it's worth researching.

If you have the time, interest, and money, you can try making the recipe twice (once with salted, once with unsalted butter) and see which tastes better. This is a bigger investment, though, so only worth doing if you really want to get the recipe as good as possible.