What is brownie brittle edges?

The crispy, almost-cookie-like crust that forms on the outer edges and corners of a baked brownie — caramelized sugar plus butter on the pan-facing surfaces produces a chewy-crisp texture that's distinct from the fudgy middle. Some bakers (and brownie lovers) actively prefer the corner pieces for exactly this. To maximize the brittle, bake 3-5 minutes past the normal done point — you'll get drier middles in exchange. Some commercial products (notably the Sheila G brand) sell brownie batter baked thin specifically to be all-brittle. At home, choosing an 11x15 pan instead of a 9x13 thins the bake enough to push the entire sheet toward brittle territory.