We bake the cocoa brownies a lot- so easy and very delicious. They should bake to an internal temp of about 185F, which is lower than the 190F+ temps that most cakes are baked to. At the point that they are done, the sides have just begun to rise and puff, but the center is still a little dipped, it won’t have puffed yet. After cooling they are flat.
It does sound like your pan is larger than called for, or it is dark or glass and needs to have the oven reduced 25F to compensate, or maybe the oven is running hot. Could any of those be the problem?