Recipe | Rainbow sponge layer cake with white buttercream icing

This rainbow sponge layer cake looks good before you slice it, but wait til you see inside!

I’ve wanted to make a rainbow layer sponge cake for ages. My birthday gave me a reason to make the effort, and it was a trial run for the one I want to make my son later in the year. I was really pleased with how it turned out.

The recipe is just basic sponge cake mix. The difficulty is in all the dividing out, colouring, etc. This certainly isn’t low on the washing up: you’ll probably use most of your bowls. But I think the finished product is worth the effort.

Ingredients (serves 10 – 12 or more or less, depending on how big you cut your slices!)

225 g/ 8 oz caster sugar
225 g/ 8 oz butter/ baking margarine
225 g/ 8 oz self-raising flour
4 eggs (medium or large)
4 different food colourings – I used orange, yellow, green and blue. You could go for a pink or purple if you prefer.

For the buttercream and decoration

200 g / 7 oz butter at room temperature
400 g / 14 oz icing sugar, sifted
red sugar sprinkles (at least 60 g)

Method

Pre-heat your oven to 180 degrees C (160 fan)/ 356 degrees F or Gas Mark 4.

I have 4 shallow, 8-inch circular tins which made making the different coloured layers of the rainbow cake a lot easier. However, if you have “normal depth” round tins, you can still use them, just put less mixture than you normally would into each one.

Line your baking tins. I’ve used my tins before and I’m confident in how non-stick they are so I just put a circle of baking paper on the bottom of each one, sticking it down with a little butter.

You can use the all-in-one-method: put all the ingredients into a large bowl and mix until combined. However I prefer to do things in stages because I think it gives you a nicer, lighter sponge.

Use an electric stand mixer or electric hand beaters if you have them. Put the butter and sugar into a large bowl and beat together until light and fluffy. Using the electric stand mixer this takes 3-4 minutes. Then add the eggs, stirring after each addition until combined. Finally, gradually add the flour, stirring gently after each spoonful until the mixture is smooth and all the flour combined.

Now the tricky bit starts!

Get out your mixing bowls. You’ll need 3 in addition to the 1 your sponge mixture is already in.

Divide your mixture equally across the 4 bowls. You can do this by eye, by spoonful, or you can be really finickity (like me) and use weighing scales.

If you plan to weigh out your mixture, a quick shortcut is that the combined weight of the ingredients you’ve used so far is approximately 32 oz / 900 g. So if you put each bowl onto the scales as you spoon the plain sponge mixture into it, you want about 8 oz / 225 g of the plain sponge mixture in each bowl.

Add a different food colouring to each of the four bowls, stirring it through the cake mixture until the colour is even. Use as much or little colouring as you like to get the shade you prefer.

Scrape the contents of each bowl into one of the prepared baking tins and use a spatula or the back of a spoon to spread out evenly. I think they look cool at this stage!

As the cakes are so shallow, they bake in 12-15 minutes. Just long enough to wash up all those bowls!

Set your timer for the lower end of that time scale and keep an eye on the cakes. When they have risen and started to brown, they’re done. If you want to be sure, stick a knife or skewer into the middle of each cake. If it comes out clean, the cake is done.

Leave your cakes to cool completely in their tins. While you are waiting, make the buttercream.

Buttercream and assembly

Use your stand mixture or hand mixer to make the buttercream. Remember, mix slowly and gently at first or the icing sugar will go everywhere. My stand mixer has a “hat” attachment which stops the icing sugar blowing up into a cloud.

To get “white” buttercream, first beat the butter on its own. With a stand mixer this will take at least 4 minutes. I think it took about 5-6 minutes for the butter I used to lose its yellow colour and go pale. Then add the icing sugar and beat for at least 2 minutes until you have a smooth white buttercream.

Put the blue cake layer on a turntable if you have one, or a chopping board if not. Put a thin layer of buttercream on top of the blue sponge layer. Place the green layer on top. Another layer of buttercream. Keep going until you have all four cake layers stacked on top of each other with a thin layer of buttercream between them. Put a layer of buttercream on the top of all the cakes. You should have something that looks a bit like this:

Use a little extra buttercream to complete your “crumb coat”. Use a knife to smooth buttercream all around the side of the cake and the top. Put the rainbow layer cake in the fridge for at least 20 minutes for this first thin later of icing to chill.

Take the cake out of the fridge and apply a second, final layer of buttercream. Work from the middle of the top of the cake, using a knife or spatula to smooth your buttercream to the edges, then down and around the sides.

Finally, add the red sugar sprinkles to the top of the cake.

Transfer your cake to a board, stand or plate for serving. At this point I had some white buttercream left over, so I piped some stars around the base of the rainbow layer cake.

It looks good like this, but the real joy comes when you slice it…

It’s delicious! And it will keep well for 2-3 days in an airtight container and a cool, dry place. I made mine during the heatwave so had to keep it in the fridge and take it out about 30 minutes before serving.