You are cooking them too long. You are cooking them at too high a temperature. You are cooking them too long at too high a temperature. Take your pick.

More heat is conducted to the edges of the brownies, where they cook more quickly. So you can take them out when the edges are done and the center is not, or you can cook them until the center is done and the edges are overdone.

We would lower the temperature at least 25°F (14°C) and see if the edges are still overdone when the center gets done. We would probably stop a little before the edges are done, in hopes that the center will catch up through residual heat.

Another option is to buy the edge-only brownie pan, which essentially does away with the center entirely, ensuring that everyone gets an edge, and that with the pan conducting heat evenly to all of the brownie, it all gets done at the same time.