It didn't smash very well. She gave the big cake a good hard whack, but it did nothing and she lost interest, so instead we gave her just a slice to play with. We had more success and got some beautiful photos, but this left me with a very big cake to use up somehow, so I took it to work and left it in the staff room with a note telling my collegues to have at it.
The rainbow cake had a fantastic reaction, and as a result I found myself making another one for a work collegues birthday, so I took photos as I went to create my very own tutorial.
The first step is to make a cake batter. Any white cake will do, I used Donna Hay's Melt N Mix buttercake. I find this recipe makes 6 cups of batter, making it perfect for a rainbow cake because you can easily divide it into three portions to colour. So first I did red, yellow and orange, and baked each one for 20 minutes in a 20cm tin. Then I repeated the process, this time with blue, green and purple.
I made 1 1/2 times more normal buttercream recipe and coloured it white, and put 1/2 a cup between each layer. Measuring it makes sure the layers are even. I like to use purple at the bottom, but that's purely personal preference.
So there you have it, instructions on how to make your very own rainbow layer cake.