I started out by making a plain victoria sponge. I like Nigellla's recipe: http://www.nigella.com/recipes/view/victoria-sandwich-3797
I doubled the ingredients and mixed it all in 1 bowl so that I could then seperate the mixture into 5 disposable round trays, that I greased with my new favourite thing, Wilton Cake Release, and poured equal amounts of the cake mixture into each.
I added a drop different coloured food paste into each of the tins and stirred until I got the desired colour. As you can see I chose red, yellow, green, blue and orange. I then baked the cakes 2 at a time for 8 mins on 180 C and left to coold for a couple of mins before turning out onto wires racks to cool completely.
The next step was to layer the slices of cake in whatever colour order you want them in, with a layer of butter cream inbetween each one (make sure this is a thin layer, or it could get really sickly). I used Betty Crocker's vanilla butter cream icing, as I ran out of time to make my own, plus it tastes delicious! Then cover the whole of the outside of the cake with buttercream.
By this time you will have a 5 layered cake covered in buttercream ready for decoration. I chose smarties as they are rainbow coloured, but you could use anything: Jelly Tot's, Dolly Mixtures, Skittles etc.
I have to say that it has a real home made/lived in feel to it, which I really like. "Gronky" our family would call it. However, when you cut into it, is where the real wow factor comes in.
Needless to say it did go down a storm, but beware, it is toothachingly sweet and I would advise against cutting yourself a huge great big hunk of it in one go, as you will probably struggle to eat it all (i know because we tried!). But this cake keeps well in an airtight container for a couple of days, so you can continue to enjoy rainbow loveliness for a few more days! Enjoy!