You will need to bake that for a long time! And it may not bake well....seems to me that cake is only leavened with baking soda....which loses it leavening power after it has fizzed....so you need to bake the cake and get the flour cooked before it falls in the middle.
The problem is also the thickness or the cake.
I would bake it 45 to 50 minutes and then test it.
I guess the best advice is cook it until it's done.
Linda C

I have baked several 12" x 18" cakes successfully. You don't want you pan to be more than 2/3 full of batter, and I lower the temp to 325. (It will take at least an hour, probably more.) The lower temp helps the center cook before the edges dry out. I always use a heating core on cakes that large. An upside down flower nail (from cake decorating supply store) or two does wonders to help the cake bake evenly. Some people also like to use 'Bake Even' strips on the outside of the pan. You can just cut an old towel into strips, wet the strips and clip them around the outside of the pan.
The Wilton website has a lot of this type of information. Good luck!
Jane

Here is a link that might be useful: Wilton website

For baking large sheet cakes I use this heating core from Wilton, works like a charm.

Distributes heat to bake large cakes evenly. Releases easily from cake. Recommended for 10 in. diameter or larger pans for complete baking.

See link below.

Lucy

Here is a link that might be useful: heating core

Sorry, I failed to mention that most craft stores will have Wilton cake supplies, such as Michael's.

Lucy

Linda, Jane and Lucy- thank you so much for the tips and advice.

My cake is baking as I type, with a flower nail in the center.

How to get a heating core to work without tearing up the cake, though? I have one and have always ended up with a mess. Perfectly done, but with a ragged hole in the middle.

Kids say it looks like the cake has a belly button!

As someone not into the Wilton cake decorating thing....what happens to the hole in the layer you must get from those cores?
If you want a cake with a big "foot print"...why not make 2 small and put them together? Seems like it would bake better.
Linda C

I've never had a ragged hole from the flower nail. I grease it well, and when you pull it out, it just leaves a hole the diameter of the nail, not the head. Easily covered up with frosting. The reason I bake a larger cake, rather than two small ones, is that is hard to have two cakes come out exactly the same size, without a great amount of trimming, which generates a lot of crumbs. Even if they are the same size, the "seam" often shows through the frosting or fondant.
Jane

The heating core is well greased inside & out, when the cake is baked & cooled 10-15 minutes the core will easily slide out; then remove the cake from the core & place it back into the hole of the cake. You may need to level, as it may bake higher than the cake itself.

Lucy

The cake turned out well, the hole from the cake nail was not bad (this really should not be a problem), but Linda, your caution about the cake's rising was timely- it doesn't look as high as it should be.

In retrospect, since this is for Valentine's Day, I should have made a couple of recipes- not combined- baking each in one square and one round pan. Split the round down the middle and place each half on adjoining sides of the square, making a heart.

I don't think this batter lends itself well to large sized cakes!

Thank you for all your help, though- I always learn so much from these forums!

I had no idea that such things existed or that they could help a cake bake more evenly. Whenever I've baked the monster, 13" diameter round cake I was afraid of overbaking the sides while the middle baked.

So the flower nail...would go upside down on the bottom of the pan with the batter poured over it? Then removed, like a thorn from a lions paw once you inverted the cake?

Nina