Yes, it is! In fact that always happens, baking a cookie cake makes somewhat of a shelf around the outside, and the edges bake quicker than the middle. I recommend two things- the first being that you cover the cookie cake with tinfoil about 15 minutes into baking. This will save it from burning on top! The second thing is to cover the outer edges (browner parts) with swirls of buttercream! That way when you are serving it, you won't even notice.