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 Chocolate Hedgehog Cake quite a few times and you can see the step by step tutorial here. I also made Thea an Arial from The Little Mermaid cake a few years ago, and you can find out how here. The rainbow unicorn cake is definitely my most ambitious cake yet, but I have found a few little cheats to make it (slightly) easier.
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 gel colour kit, which I found in Sainsbury’s for £2.25. It has 5 colours of gel food colour to make the 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.
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.
The baked cake comes out only slightly darker than the raw mix.
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.
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.
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.
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 piping gun like this one*, but an icing bag would also be perfect.
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.
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 ears, horn and eyelashes for £11.99 from Amazon and they are perfect, and reusable*.
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.
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.
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.
We all had a fab time at her unicorn themed party.
I hope you like my rainbow unicorn cake tutorial, why not Pin it for later?