It was delicious! But my cake sank quite a bit as it cooled, which made slicing it in half a bit of a challenge. Any idea so?

3 ideas
1) if cake is baked through & you haven't in fact sliced in half, use cream designed to go between layers as a filling for the sunken part. Dust with some confectioners sugar & serve.
2) look at this article for possible causes contributing to low height or sunken result
https://www.craftsy.com/blog/2014/09/how-to-keep-a-cake-from-falling/
3) wait to hear from cookbook & recipe author