This is the cake I made for my daughters boyfriend's 21st.

It was a rainbow cake that I had seen on the Internet a few times. The first one I made was for my son's 21st. They were really pleased and surprised with it so I thought I would do another. It is a basic Victoria sponge recipe but with the ingredients trebled.
Cream together 12oz margarine and 12oz sugar until it is white and creamy. Gradually add 12oz self raising flour and 6 eggs alternatively
Divide between six bowls.

.Add gell food colouring Red, yellow, blue, green, purple and orange. Gell colouring is better as ordinary bottled colouring dilutes the mixture too much. Use an ice cream scoop to divide the mixture equally into the sandwich tins. I used two silicone "tins" I bought from the pound shop giving them a quick spray of 1 calorie oil before putting the mixture in.Bake at gas mk 5 for aprox 20 minutes.
When cooked and cooled sandwich together with butter icing (8oz butter mixed with 1lb icing sugar). Just put a thin coat on each layer as there will be five lots of butter icing between the layers as well as all over the cake. Cover the cake with the rest of the butter icing.

To decorate the cake I cut the numbers "21" out using my Cricut but I used the negative placed on top of the cake and, placing a piece of paper diagonally over the stencil, I sprinkled the first colour on. I then moved the paper about a sixth of an inch over and sprinkled over the next colour and so on.
 I then had the choice of either picking off the few sprinkles I had accidentally spilled or adding a few more to the cake. You can see what I decided to do! I put 21 candle holders around the cake and bought some rainbow candles from Asda. They are not on the cake as it had to travel. I haven't seen the inside of one of the cakes yet, although I am told there is a picture somewhere!