Last week for my birthday, DBF made me a cake...his first from scratch! He was so proud of himself, and so was I. It was delicious, but he was disappointed that when he took the pans out of the oven, the center of each layer was a bit sunken and dense, even though they were cooked through. I've never had this problem. I know that over-mixing or too-hot an oven will cause a cake to mound up in the middle. What would cause the center to sink in?