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hardlikearmour is a trusted home cook.
Fleur de sel is sea salt, typically coarsely ground. Popcorn salt would not make a good substitute as it is very fine. I'd go with kosher instead if you can't find fleur de sel.
Fleur de sel is a form of salt that is collected (according to Wikipedia!) in a special way, particularly in Brittany. It has a higher mineral content than most salts and can have a pinkish color. I think due to the minerals it has a slightly more complex flavor than most salts.
Maybe it's because I am a poor graduate student, and others may disagree, but I think that you're absolutely fine subbing regular salt. If you have kosher salt, fleur de sel, or other good salt around, they are great and I definitely use them when they're available (read: at my parents' house). I think that they probably do add something to the food. But I'm pretty sure that you won't notice the difference if you substitute regular salt.
Let me amend that: if you're cooking and subjecting it to heat, you're usually going to dissolve or change the texture of the salt crystals anyway, and I don't think that the grind of the salt would matter. If you're sprinkling the salt on top of something, like a dessert, then I agree with hardlikearmor that the texture will be messed up, and you should try to find a more coarsely ground salt.
The other issue is that more finely ground salt has more "salt" per measure than more coarsely ground salt. For instance Diamond kosher salt weighs half of what table salt weighs for the same volume. If you're measuring by weight, then salt is virtually interchangeable. If you measure by volume you may end up with an over or under salted final product.
Popcorn salt is extremely finely grained, almost powdery - the opposite of fleur de sel and Kosher salt, which are both large, chunky crystals.
Cynthia is a trusted source on Bread/Baking.
Salt is salt is salt isn't quite true. Look over the received wisdom above. I spent some years as a poor grad student also. Kosher salt is fairly easily come by and a good middle of the road between Morton's and whatever. It's free of anti-caking additives that give table (or popcorn) salt that chemical "bite". World Market, Whole Foods, a good grocery store are all good sources. But popcorn salt, no. You really won't like the result.
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