Rainbow Cake with Petal Icing and Smartie Surprise.

Rainbow cake feature

This year, for Littlest’s 4th birthday, I attempted my most ambitious cake to date. She requested a rainbow cake with rainbow icing and a rainbow on top. I decided to make a six-layer rainbow cake with rainbow petal buttercream, a fondant rainbow on top AND Smarties inside the cake for that extra rainbow surprise.

To see how I decorated the cake, take a look at our video. Below, you will also find written instructions and information about the materials for making the cake.

Video Tutorial

Making the Rainbow Cake

You can use this technique to decorate any kind of cake. However, if you wish to create a six-layer rainbow, there are many recipes online to show you how. Essentially you will need to make six, 9-inch flat sponge cakes and colour them with gel colours for a more vibrant hue. We use Wilton gel colours. 

Stacked 6 layer rainbow cake without icing

If you are pressed for time or simply feel a bit overwhelmed at the thought of making six cakes, you can use cake mix to make life easier! Most mixes will make two 9-inch cakes.

Rainbow Cake Decoration

Finished Rainbow Cake

You will need:


Smooth the plain buttercream all over your stacked cake using a piping bag and large palette knife. You will need around 200g of buttercream to cover the cake fairly thinly. You should aim for a really smooth finish. Once covered, allow to chill in the fridge for as long as possible – at least an hour.

Meanwhile, prepare six piping bags with six different coloured sets of buttercream. You will need around 150g -200g of buttercream per colour. Then, divide into six portions and mix in the gel colours until each set has a uniform colour. Next, spoon into the bags. (The easiest way is to use a pint glass and spread the edges of the bag over the sides of the glass.)

Snip the end of each pipe with a fairly large hole. Try to ensure that each colour has a similar-sized opening. Tie the tops of the bags in knots and you are ready to ice.

Icing the Cake

Remove the cake from the fridge. Using a toothpick, mark the half-way point on the cake. This will guide your icing. Starting with red, make six round ‘blobs’ of icing in a straight line from the top edge to the bottom edge of the cake. Follow the colours of the rainbow. Smooth each blob into a petal shape with the back of a spoon, taking care to wipe in between colours.

Inside the rainbow cake - a Smartie surprise.

On the tail of each ‘petal’ repeat the blobs of icing, this time starting with purple and then moving red. Then, smooth into petals again. Next, start with blue and follow on with purple and red. Follow on in this way so that the petals create a pattern. Carry on until you have iced all the way around.

Next, remove the middle section of the cake using a cookie-cutter and fill with Smarties or any other rainbow sweets you like. Return the red cut out to the top of the cake and smooth over the top with buttercream.

Create one more set of ‘petals’ around the top edge of the cake. Using white frosting (to look like clouds), pipe flower shapes in a circle on the remaining exposed part of the cake. Sprinkle the ‘clouds’ all over with rainbow coloured sprinkles.

Finally, create a rainbow using fondant icing. I used six colours and rolled them into tube shapes. Next, I then created the rainbow on a small round shape (like a shot glass). Lastly, I used white fondant and a cloud cutter to add to each end of the rainbow. You can use water to stick the different fondant parts together. Once I had placed it on the centre of the cake and pushed firmly to check it was secure, my cake was ready.

Littlest was really thrilled with it, so it was all worthwhile!

If you liked this post, you may also like:

Why do I Make Complicated Birthday Cakes?

Moana Cake Tutorial

Please note, this post contains affiliate links. This means that if you click through and buy something, we earn a small commission at no cost to you. For more information see our Disclosure Policy.

Pin for How to make a rainbow cake

If you have enjoyed this post and found it useful, here are some ways you can support Someone’s Mum:

Buy us a virtual coffee

Follow Someone’s Mum or Daddy Cooks Food on Instagram

Follow Someone’s Mum or Daddy Cooks Food on Facebook.

Share this post with your friends!

Thanks so much for your support.

2 thoughts on “Rainbow Cake with Petal Icing and Smartie Surprise.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.