Rainbows and sprinkles – how to get a rainbow stripe on the side of your cake

One of my favourite things about cakes by commission is that every single one is different.

For this bright and cheerful Trolls birthday cake, rainbow icing was the only way forward! Decorating your cake with a rainbow may seem daunting, but it’s surprisingly easy, with just a few steps.

To decorate an 8 inch cake you’ll need:

  • 600g Icing sugar
  • 300g Unsalted butter or block margarine
  • Food colouring for your rainbow colours
  • Icing bags
  • Flat or Angled spatula

To start, with clean hands, weigh out your unsalted butter/margarine into the bowl of a freestanding mixer and beat until creamy, smooth and pale.

Next weigh and sieve your icing sugar into a separate bowl, slowly adding bit by bit until completely combined. I also like to add a splash of vanilla essence and a splash of milk to my icing, for flavour and texture.

When your butter icing is made split into 7 portions – 1 to crumb coat your cake and the others to individually colour with your food colouring.

To crumb coat your cake, use your uncoloured butter icing to completely cover the cake with a thin, smooth layer of icing, using your spatula to smooth the icing along the edges and remove any excess to create a smooth line. Keep scraping the excess from your spatula off into a bowl, and keep this extra icing handy for decoration. Once you’re happy with your crumb coat pop the cake in the fridge to chill.

Whilst your cake is chilling colour your remaining 6 portions of icing: red, orange, yellow, green, blue and purple. Once mixed put each colour icing into its own piping bag and lay on the counter ready to ice your cake.

Take out your cake and pipe a strip of each colour starting with purple and working your way up to cover the cake, ending with red. Pipe a good thick layer of each colour before smoothing with your spatula, regularly scraping off the excess and continuing the smooth until you have clean, sharp edges.

You may find this easier with a turntable for your cake or a lazy Susan if you have one to help you get a better finish.

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