Rainbow Cake

A couple of days ago I made made a birthday cake for my boyfriend and his friend for their 20th.
This was the perfect opportunity to do something totally over-the-top! and to surprise them. We called it a troll cake just for fun to keep the rainbow a secret.

Marie and I used:
  • 2 x Edmonds 'butter cake' cake mix, with the milk and margarine added
  • food colouring in red, blue, yellow and green
  • 250ml of cream
  • 750g of white chocolate buttons
Chocolate ganache was used to ice this cake. I prefer ganache as its not as rich as buttercream. I'd made milk chocolate ganache before but I thought white would be particularly pretty for this rainbow cake. It is essentially just chocolate and cream, mixed together thoroughly to form a paste which can be used for cakes and desserts. It makes a great alternative to fondant icing - which I personally find way to sweet.

To make a white chocolate ganache, you need to use 3 times as much white chocolate as you use cream. So if I use 300ml of cream, I need 900g of white chocolate. This is because white chocolate is much softer than dark or milk chocolate and so needs less cream to turn it to the right consistency. 

For the ganache:
  • Take a large microwavable bowl and add the 750g of the chocolate buttons and 250ml of cream. 
  • Stir the lot so all the buttons are covered in cream.
  • Place the bowl in the microwave and heat on high for 1 minute.
  • Take it out and twist the bowl around to loosen up the mixture. do not stir at this stage.
  • Continue to do this in 1 minute intervals until you see the chocolate start to melt into the cream.
  • Take a metal spoon or a plastic spatula and start to stir the mixture.
Never ever use wooden spoons when using chocolate - the chocolate gets into the wood and the chocolate stays warm - just a recipe for disaster.
  • It should be very runny and thoroughly mixed.
  • Make sure you do not burn the chocolate, if burned, it cannot be saved!
  • Leave the chocolate to cool to room temperature and then put it in the fridge for a few hours while you cook the cake. This will allow air bubbles to escape and for it to get thicker so you can spread it later. 

For the rainbow cake:
  • Empty the 2 cake mixtures into a large mixing bowl - for this one I needed to add some milk and margarine so I did that also. 
  • Follow packet instructions until the mixture is thoroughly whisked and mixed.
  • divide the mixture between 6 bowls and then...
  • go crazy with your colours so make them as bright and accurate as possible - however you'd like really :D start off with small amounts of food colouring though, as too much will be very dark and this is non reversible!
  • take a few cake tins of the same size if you have them, or it not then use one cake tin and cook all the cake mixtures individually once each one is cooked.
  • because there isn't much mixture in each colour, they don't take long. I cooked each of mine at 190C for 20 minutes each.
  • allow them to all cool completely
  • find a cake board or plate to serve the cake
  • take the white chocolate ganache out of the fridge and start layering!
  • start with purple and layer up in the order of a rainbow, I spread a very thin layer of white ganache just to help the cakes stick together
  • once all placed on, start coating the entire cake in the ganache. You may need to put it in the microwave for 10 second intervals until you can stir and spread it easily.
  • Devour by yourself or with others :P
The rainbow cake was a hit - we called it our Troll flavour cake and so the rainbow surprised them!


  1. OMG! THIS IS AWESOME! like the pics are amazing...can't wait for Friday! you have me hooked xx

  2. Thanks! yes Friday was good and I've posted it up :)