It's by Vanessa Carlton. Beyond being adorable and very pretty, she also seems to be a very lovely sweetheart and a nice person. I read an article about the production of this song. She went into the studio to watch the orchestra record their piece and she said she was taken a back and a little embarrassed watching them play music she wrote in her livingroom in her pyjamas.
https://youtu.be/Cwkej79U3ek