Children’s book including a migrating beauty mark, magic, and alicorn horns

I remember this book being larger than standard books.

Details I remember:

•the art was beautiful

•a young boy was the main character

•there was a witch (who I think used to be an actress ) who’s beauty mark would migrate to different spots on her face

•there was a black cat

•they needed an alicorn horn for something and it was revealed that it was being used as antenna on the top of the roof