I'm upset to say the least, I have no funds to go buy more cake ingredients again and I really need to have it done today! I make a 12" round chocolate cake and made it as usual where before today I haven't ever had this issue but I normally make nothing bigger than a 9" round. Why did it sink? It was completely baked in the center and was BEAUTIFUL when I pulled it out of the oven but immediately upon setting it down on the stove top to do the initial cooling it sunk before my eyes. I was able to get it out of the pan and it's cooling further on a rack with a HUGE indentation in the middle. I am just sick!

What happened?

A cake usually sinks in the middle because it isn't completely baked.

It was undercooked. You need to bake the cake with a heating core in the middle of the cake when it's that large.

Don't but the core they sell for $8 in stores. I use a #7 flower nail turned on it's head and it works great for $1.49.

But, the tester came out completely clean I ran to the store and got nails for the next one; I wonder if that was part of the problem?

I have had that happen when I used a flower nail too. Use the flower nail and even if the cake tester looks clean, make sure the cake springs back when you touch it and also the sides of the cake are starting to release from the pan. I also use Magic Cake Strips which will even out the cooking.

Cakes can sink in the middle for other reasons beyond being uncooked.

Your cake may have sunk because you opened your oven before the cake was set (set and done are two different things), over whipping the batter, liquid/fat/sugar or leavening in your recipe was off and/or some recipes do not double or triple to make a larger batch well.

If the center is fine and really completely cooked you should be fine. If you have to level your cake and after that is done there is still a small sink hole in the middle, just take some of the scraps and fill it in.

It's true, you can't just double or triple cake recipes to fill a pan.

You might want to check out The Cake Bible, in the back where she tells how to increase her wedding cake base recipe to whatever size pans you need. I don't have the book with me right now, but she explains her method for calculating the amount of leavening needed based on the surface area of the pan in great detail.

Baking is more science than art.

I would be most interested in hearing people's experiences with using the Heating Core in cakes over 9 inches diameter.
Am especially interested in 10, 12 and 14 inch square cakes.
Many thanks.

As was already noted cakes sink in the middle for a variety of reasons--you cannot assume it sank simply because it was underbaked.

If it was baked completely through as your tester demonstrates then you need to consider other reasons. First you have to consider why would it sink if it was baked completely through? Your answer is that it is a structural issue--the cake cannot support its weight. There are a number of things that can affect the structure of the cake (some due to the recipe others due to how it is mixed) which cannot be solved by simply adding a heating core. For instance a heating core will not address an unstable structure produced by too much leavening agent.

If it was completely baked I would suggest looking at things other than a heating core. If you simply doubled or tripled a recipe optimized for a 8 or 9 inch pan, you should look at the amount of leavening. If it is a recipe optimized for a larger pan, consider how it was mixed. Unfortunately there is no easy answer.