Sonia, it's basically because you might have just a bit too much batter for your pan. As the cake rises, it eventually loses its side support (the pan), so it has nowhere to go but up and towards the center, since it's set enough not to simply overflow the pan. It's also, as you say, the outer edge baking more quickly than the center, and reducing the oven temperature might help a bit; I've done that experiment. But it's mostly simply a case of excess batter for the pan size. PJH

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