Hard edges on brownies usually can be blamed on the type of pan you use or on overbaking. When I use my heavy-weight aluminum pan, and take care not to overbake, my brownies are perfect every time.

For various reasons recently, I've used both a disposable foil pan and a cheap steel pan for two different batches; each time, the corners and edges were very hard, almost crisp. A glass pan can also be responsible for the problem. (When using glass, the oven temp should be set 25° lower than for metal pans.)

When brownies are baked for the proper time, they may not look completely done in the center, but they'll firm up after cooling. So if you're trying to make the center look "done" before you remove them from the oven, that probably is at least partly responsible for the hard edges. When testing the brownies for doneness with a wooden pick, it shouldn't come out completely clean, but should have crumbs clinging to it.