Heya.
the story was called, interestingly enough, "Carrot Cake " :p You can most easily find it in a scary story collection called "The Unseen" by Janet Lunn. I was looking for this book when I came across your question (Carrot Cake was my favorite story in it when I was a kid), so I had to register and let you know once I had found it, as I know how maddening it is when you can't find something you really wanna know

Hope you can track it down.
Peace.