Ok I am about to SCREAM!!!!!! I had to bake a 11x15. I did the cake and the middle was moist and wonderful but the corners were very dry and almost burnt. My first customer (of course) said everything was great but the corners. What do I do? I also made a barbie cake and it was a complete disaster....Burnt, Burnt, Burnt, by the time I cut all the burnt off there was not much left so there was no barbie cake. When I check on my cake the inside is always undone so I have to cook it longer to get that done which affects the corners....HELP I JUST WANT TO CRY!!! 
I agree with quadcrew. Ovens are so vastly different. I've baked in all different kinds of ovens..convection, gas, electric. Calibrating your oven is very important
If you are so inclined, do a test bake with cupcakes. Fill your oven, putting pans on each rack and don't rotate anything during the bake. This will show you exactly where your hotspots are. I know it sounds like a waste, but then you'll know exactly how YOUR oven works. If nothing else, you'll get some cupcakes to snack on 
This weekend I tried using a flower nail in the center of a 9 x 13 and a 10 in round...it worked great for both of them. On my round cake, I also use the bake even strips...I have to say it turned out MUCH better! The strips help to keep the edges from baking too quickly, and the nail helps the center to bake. HTH
YAYA
I agree with all of the above. Buy an oven thermometer. I have a convection oven and bake at 325deg. Also, use a flower nail or 2 in a cake that size. Grease it well, turn it upside down in your pan and place in the middle of your pan.
Jacqui
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