How did I not know about this? I was alerted to a post over at HPProgs that a small prequel that JK Rowling wrote for a Waterstone charity benefit is now online. You can find the entire excerpt that she wrote on the HPProgs website. Waterstone also published a small anthology of the pieces from all the authors that contributed such as Neil Gaimon, Margaret Atwood, Nick Hornby, and more!
Here's a small teaser from the excerpt transcript: