Tips before you start: be prepared to make three batches of cake mix in a go. One cake mix makes 2 sponges. It's better to do two cakes at a time because it's easier to divide a mix in half than it is into 6, and also you don't end up with cake mix on your kitchen ceiling - a possibility if you have a stand mixer as strong as mine. Anyway once you've done one cake mix and the two cakes are in the oven, get started on the next one. Carry on like this and dance to some music while you do it. I find Disney Enchanted 'Happy Working Song' does the trick. I'm not even ashamed to admit it.
Oh and FYI you don't have to use an electric mixer at all but due to the volume of cake it did make my life easier...
And one last thing: make sure you keep track on your colours. I found it easier to do the cold colours (blue, purple, green) first and then the warm colours (yellow, orange, red).
Cake: 3 quantities of the following:
6 oz / 150g caster sugar
6 oz / 150g margarine
6 oz / 150g self-raising flour
3 eggs (beaten)
1 teaspoon vanilla extract
*food colourings: red, blue, yellow (you can use these to make the purple, orange and green - or just buy these too)
20 oz / 500g icing sugar
10 oz / 250g soft unsalted butter
4-6 tablespoons milk
1 teaspoon vanilla extract
18 oz / 450g icing sugar
2 egg whites
1 teaspoon glycerine
2 teaspoons lemon juice
Equipment: 2 circular cutters: 5 cm and another 10cm approx.
1) For the Sponge Cakes: Grease and line two sandwich tins and set aside. Preheat the oven to 180C /170 Fan/ Gas 4. Cream the margarine, caster sugar and vanilla extract together until pale and fluffy. Gradually mix in half the beaten eggs, only adding a little at a time to prevent the mixture curdling. Sieve half of the flour into the mixture and lightly beat in. Repeat with the rest of the eggs and flour until all cake ingredients are mixed together. Place half the mixture into another bowl and tint with your colour of choice. Bake the sponges in the middle of the oven for 20-25 minutes, until springy to the touch and an inserted skewer comes out clean. Turn out onto a cooling rack. Repeat twice more with the other colours. Once cooled trim the brown crust off the sponges to reveal the bold colours underneath.
2) Make the Buttercream Icing: Beat the butter, vanilla extract and a little of the milk together to soften everything together. Gradually sieve in a little icing sugar at a time, beating after reach addition. Continue this until all the sugar has been incorporated and the icing is light and spreadable.
3) Cut the cakes: Align the warm and cold colours next to each other as the picture above. With the circular cutters, cut each cake into two concentric circles and one small circle from the centre. Carefully mix and match the pieces of cake so there are three different colours in each circle. Repeat this process until no ring or circle is next to another of the same colour. Once you're happy, stick these in place with butter cream Aim to use no more than a quarter of the buttercream for this.
5) Make the royal icing: Whisk the egg white until frothy - but not stuff - sieve a couple of tablespoons of icing sugar into the egg whites, adding the glycerine and lemon juice. Beat until smooth. Continue to sieve in the rest of the icing sugar, mixing after each addition, until a thick but spreadable consistency has been achieved. If it's too runny, add more icing sugar and if it's too thick add a dash of water. Cover with a cloth for 20-30 minutes to allow any air bubbles in the icing to disappear. Take 5-6 tablespoons of icing and place in a separate bowl for decorating the cake later.
6) Decorate the cake: Spread the royal icing onto the top of the cake, allowing it to cascade over the sides. Then continue to spread the icing around the sides until the whole cake is covered. Allow to dry for an hour or so before decorating
7) For the decoration: Divide the remaining icing into 6 separate small containers like a mug and colour each little portion of icing in the colours you used for the cakes (blue, green, purple, yellow, orange and red) Place these into 6 separate disposable piping bags and pipe a rainbow style boarder around the cake (use your own imagination with this ;) ) I dusted mine liberally with glitter because...well why not? Allow to set before amazing everyone with this technicolour dream cake. Yeah that's right, I made THAT reference.