Casual Kitchen -   Rainbow Cake

Rainbow Cake

20th November 2016

Oh boy, let’s talk Rainbow Cake. Once you get over the initial intimidation of the sheer size of this cake you’ll come to realise, it’s just 6 simple sponges. Decorated with patience. A lot of patience.

I decided to make this after an okay attempt with my friend Debs the previous week. We went in a little unprepared so being obsessive that I am, I decided to have another bash at it. Rather than a cream cheese frosting I opted for a buttercream to give it a stronger structure as the cream cheese was a little melty.

As mentioned, the recipe itself is rather simple – just a little bulky. You may need to split the batter into two batches as I struggled with my standard KitchenAid. So let’s crack on.



  • 10 tbsp whole milk
  • 1½ tsp vanilla extract
  • 750g soft unsalted butter
  • 750g caster sugar
  • 9 large eggs
  • 750g self-raising flour
  • Food colourings (I prefer the pastes)


  • 400g soft unsalted butter
  • 900g sifted icing sugar
  • 4 tbsp whole milk
  • ½ tsp vanilla extract


  1. Preheat your oven to 180 degrees celsius.
  2. Cream together your butter and sugar until fully combined.
  3. Add your eggs one by one along with your vanilla extract. Keep this going on a medium speed until you have a smooth batter.
  4. Meanwhile, sieve the flour into another bowl. Drop the speed on your mixer and add the flour. You may have to do this in stages or stop and start your mixer. Be careful not to over mix. As soon as the flour is fully combined, you’re done.
  5. Here’s where a little judgement comes into play, if the mix is too thick (i.e. it’s taxing to stir) combine the whole milk to loosen the mix.
  6. Divide the mix into 6 bowls, add the colourings and mix by hand.
  7. When done, spoon the mix into a lined 20cm baking tin and bake for 20 minutes in your pre-heated oven. Remember to do the skewer trick to ensure your cake is cooked all the way through before taking them out of the oven.
  8. Once baked, remove the sponges from their tins and let them cool on wire racks.
  9. While your sponge cools we’ll crack on with the buttercream. Start by putting your buttercream in your mixer on a medium-high speed for at least 5 minutes. This might seem like a long time, but you need to do this to whip in as much air as possible. This lightens the butter and makes for a whiter buttercream.
  10. Sieve your icing sugar and slowly add it to your butter. You’ll need to drop the speed on your mixer down to the lowest possible setting to avoid a sugary explosion!
  11. Once all of your sugar is added mix the vanilla extract and milk together and then add to your buttercream. By now, it should be light, bright and white.
  12. Cover it clingfilm and place in the fridge.
  13. Take your sponges and level them off so that they are completely flat when stacked. For added neatness you can always cut a perfect circle from the sides as well.
  14. When you’ve neatened your sponges, stack them and place them in the fridge to harden. Leave them for at least 1 hour.
  15. Once chilled, take your sponges and buttercream out of the fridge. Place a small dollop of buttercream on your cake board and then place your first sponge on.
  16. Then add about two wooden spoons worth of buttercream to the top and using an offset spatula, give the sponge a tight crumb coat. Once fully coated, add a generous amount of buttercream and smooth out a ½ cm layer.
  17. Add your other layers, repeating the process for each. It will look a mess but don’t worry, that’s expected.
  18. Finally, take your remaining buttercream and apply a generous amount to the top and smooth out all over the cake. This will take patience and persistence to fully smooth out. I find the use of a cake scraper is a valuable addition when at this stage.
  19. Once smoothed, decorate as wished but I left mine completely bare to really surprise people when they first cut in and saw the bright layers within.
  20. Enjoy!