How to Make a Super Cute Rainbow Unicorn Cake

Have you seen the amazing unicorn cakes all over Pinterest and Instagram? Thea spotted a rainbow unicorn cake on my Instagram feed, and wanted me to make it for her fifth birthday. Luckily, I am up for a challenge!

I try and make the girls’ birthday cakes, like my mum, though I don’t always manage it. I’ve made a I also made Thea an . The rainbow unicorn cake is definitely my most ambitious cake yet, but I have found a few little cheats to make it (slightly) easier.

How to make a cute rainbow unicorn cake

But a note of caution: although the actual cake is not complicated, it is time consuming, so please make sure you have plenty of time! I estimate it took me about five hours altogether. Maybe more. I highly recommend making the cake, and the icing a day or two before it’s needed and leaving the final decorating until the next day.

This unicorn cake would also look just as good with three or four layers if you didn’t want to go for the rainbow effect, and it would be cheaper to ice!

What You Need to Make a Rainbow Unicorn Cake

  • 5 x 7 inch cake tins with the bottoms lined — straight edges are best
  • 1 set of five gel colours
  • 8 large eggs
  • 440g softened butter/Stork
  • 440g caster sugar
  • 440g self raising flour
  • A few teaspoons of lemon juice to taste (optional)

Pre-heat your oven to 180°. You will need five seven inch cake tins, unless you wait for two to cook, then refill the tins. I would recommend borrowing them if you can though, as it will take much longer, and you want to get cakes in the oven as soon as possible after adding the eggs. Grease the tins and line the bottoms with baking paper. Try and use ones with straight edges to give a straight edge to your finished cake.

Make your cake mix by creaming the softened butter or Stork and the caster sugar together. I used a food processor which makes it much easier.

Add in 4 large eggs and half your flour. Beat until smooth and then add in rest of the eggs and flour and beat again. If you like, add some lemon juice to taste.

Colouring Your Rainbow Unicorn Cake

Split your cake batter equally into 5 bowls and add the food colouring. I used this little . It has 5 colours of gel food colour to make the rainbow cake.

Rainbow food colour to make a rainbow cake

The recipe says to use it all but that gives a very bright effect so I used about half as I wanted a pastel effect. I erred on the side of caution, adding a few drops at a time, until I got the right shade.

Add food dye to cake batter to make a rainbow cake
Rainbow cake mixtures ready to go into the tins

Put the cake mix into the tins and bake for 35–40 minutes. They will go slightly golden on top and you can’t see the colour that well. But don’t worry, they will look great when they are sliced up. You can see how mine looked when they came out of the oven, and how they looked when they were sliced.

Freshly baked rainbow cakes
Rainbow unicorn cake sliced up

The baked cake comes out only slightly darker than the raw mix.

Rainbow cake mixtures ready to go into the tins
Slice of rainbow cake

Let the cakes cool completely before you assemble it.

How to Ice Your Rainbow Unicorn Cake

You will need a huge amount of buttercream icing. I would recommend making more than you think you need, as it’s really frustrating to run out and have to mix up more. You can freeze any leftover icing for up to three months, so it won’t go to waste.

For your base icing layers:

  • 1kg icing sugar
  • 500g butter
  • A few teaspoons of lemon juice to taste (optional)
  • Pink or red food colouring

Add the butter to your food processor or bowl and beat until smooth. Gradually add the icing sugar, sifting if if you are mixing it by hand. Add any flavourings and a few drops of food colouring. You want a very pale pink for the base layer so go easy!

Stick the bottom layer of cake onto your serving plate using a little icing. Then spread a layer of icing on top of the cake, making it thicker at the outer edges, to make the cake sit evenly.

Repeat for each layer, working from blue to green to yellowy to orange to pink, making sure it’s as straight as possible.

Use a small amount of icing to stick the base cake to the plate
Use butter icing to sandwich your rainbow cakes together, starting with green at the bottom, then blue
The stack of rainbow cakes, sandwiched together with butter icing

Chill the cake for half an hour or so in the fridge, to help the icing set.

When it’s cold, use a spatula to cover the whole cake in a thin layer of icing. Fill in any gaps around the edges so that you get a flat-ish finish. This layer catches all the crumbs so that the next layer doesn’t get them mixed in.

The crumb layer of icing on the rainbow unicorn cake

Chill again for half an hour, and then add the top layer, getting it as smooth as you can. I didn’t worry about a bit of texture but you may want to get it smoother.

The top layer of pink icing on the rainbow unicorn cake

Chill again. I left it in the fridge overnight.

How to Decorate Your Rainbow Unicorn Cake

You’ll need to make a tonne more icing now. I lost track of how much I made, but I think making the same quantity again as you made for the base layers of icing should be enough.

Mix it up and then put it into several bowls, one for each colour you want for your unicorn mane. Three or four colours should be enough, using the leftover gel colour from the kit, or some purple and pink food colour if you prefer. I left a small amount of icing with no colour for highlights, and made three shades of pink. I tried to make purple by mixing red and blue dye, but it went rather grey!

Start with the darkest pink and pipe large rosettes in the centre of the ‘forehead’ and scattered around the top and down the back for the mane. I used a *, but an icing bag would also be perfect.

Pink icing rosettes on a rainbow unicorn cake

Refill your icing piper with the next shade of pink and add some smaller roses.

TIP: If you have a piping kits you can fill each one with the different shades of icing, which would make it much quicker.

Two tones of pink icing rosettes on a rainbow unicorn cake

Next, add your horn and ears. You can make them out of sugar paste, but I think life is too short. I got my *.

Unicorn horn, ears and eyelashes for unicorn cake

Next, add the palest pink icing to your icing gun and pipe little roses scattered over the top and mane, and around the ears and horn, to cover the plastic.

Back of a rainbow unicorn cake
Unicorn cake with the horns and ears

Pipe on a few blue or purple roses and finally add your cream coloured ones, making sure the gaps are all filled, and the mane curves round to the side.

Top of a pink rainbow unicorn cake

Finally, use some icing to stick on your eyelashes, and (carefully) put in to the fridge until party time. You could add a dusting of edible glitter or sprinkles if you like too.

What Did the Birthday Girl Think to Her Rainbow Unicorn Cake?

Thea was over the moon with her rainbow unicorn cake, and was super impressed when we cut it open to reveal the rainbow effect.

Birthday girl with her rainbow unicorn cake
Inside of the rainbow unicorn cake

We all had a fab time at her unicorn themed party.

Kids party with a rainbow unicorn cake
Rainbow Unicorn Cake with gold candles

I hope you like my rainbow unicorn cake tutorial, why not Pin it for later?

How to make the cutest rainbow unicorn cake with a few simple cheats to make it a little easier

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