This Buttercream Rainbow Cake has a hidden Rainbow buttercream centre it is also decorated with the cutest Rainbows.

This is a bit of a different post compared with my normal recipes. I am going to share a step-by-step guide to making this beautiful rainbow cake.

I have been experimenting a little bit lately with decorating my cakes and I have been having so much fun experimenting. Recently bought a lot of new cake decorating equipment which I will go through in a new post, but for now I hope you enjoy this Buttercream Rainbow Cake.

Remember if you make any of my recipe share them on social media by tagging me or using #aspoonofvanilla.

Jump to Recipe

Let’s Start with the Cakes

Okay, so for the cakes I used 4 Vanilla Cakes. I increased the recipe for my Vanilla Cake to add in here….

You could make the cake a three-layer if you prefer but I just prefer the height of four layers for this cake. You could also make this cake chocolate using my Chocolate Drip Cake, Rainbow Cake or Cookie Dough Cake it is completely up to you.

Once you have baked your four cakes leave them to one side to cool completely.

Rainbow Buttercream Centre

So, for this cake it may seem like you have a huuuge amount of buttercream but don’t worry because we are going to be splitting it into individual bowls to make lots of different colours for this cake.

To make the buttercream just mix together the Unsalted Butter, Icing Sugar and Vanilla Extract. I wouldn’t recommend using a baking spread like Stork for your buttercream as it will just make your buttercream really soft and sloppy, and it won’t be able to hold up your cake.

Rainbow Drip Cake

Vanilla Cake

Cookie Dough Showstopper Cake

For the buttercream you need to split it into seven bowls (sorry! There is a lot of washing up with this cake but trust me it is worth it!). The difficulty with this part is that you don’t need the same amount in each bowl. You will need enough dark blue to decorate the outside of your cake with you will then need a fair bit of red as you are going to pipe this around the edge of your cake when stacking them to create a rainbow in the centre.

For your rainbow colours you will then need a little bit less of each colour. So, you will need the most amount of red, then yellow, orange, purple and turquoise.

You will also need to leave some buttercream white for the crumb coat and clouds, the amount you want to leave white is completely up to you. I used the white to look like clouds on my rainbows and I then also decided to pipe the top of the cake to look like rainbows too but as always this is completely optional.

Colouring your buttercream

To colour my buttercream I used Pro Gel Food Colouring. I would recommend using gel food colouring as you don’t need a lot to get a really vibrant colour. For this cake, I used red, yellow, orange, purple and turquoise for the rainbow and for the crumb coat and sides/top of the cake I used blue food colouring.

For this part of the cake place all the colours into piping bags with a large round nozzle. You won’t use all of the buttercream so any leftover just pop back into your bowls for later.

Stacking the Cakes

Place your first cake onto your cake board. Using the red buttercream pipe around the top edge of the cake. Next to the red now pipe a circle of orange, then yellow, purple and when you reach the centre pipe the blue.

Now you will see why we need more red buttercream as opposed to turquoise as the red has to go all the way around the outside edge of the cake whereas the turquoise is just a tiny little circle in the centre.

You should now have a rainbow on top of your first cake. Place your next cake on top and repeat the process until your cake is stacked.

Using the white buttercream go around the sides of your cake and fill in any gaps. Using your cake smoother smooth the buttercream out. Don’t worry if you can still see the cake through the buttercream this is just our crumb coat, we will completely cover the cake with the blue buttercream.

Place the crumb coated cake into the fridge for around 30 minutes or until the crumb coat is completely firm.

Decorating the Cake

Once your crumb coat has firmed up it is time to decorate!

Using your blue buttercream pipe around the side and top of the cake. Don’t worry this doesn’t have to be neat and it doesn’t have to be perfect as we are just going to smooth it out.

Smooth the buttercream first with a palette knife. This will take a little of the excess buttercream off before we use our cake smoother.

Once the buttercream has been smoothed slightly with your palette knife go around the sides of the cake with your cake smoother and smooth. If there are any gaps fill these in with buttercream and smooth again.

If any of the buttercream comes off the cake just pop this back into the bowl.

To finish the top edges of the cake using an angled palette knife gently smooth the edges back into the centre of the cake. Clean your palette knife every time after smoothing as you don’t want to put any of the buttercream back onto the cake.

Pop the cake back into the fridge for around 30 minutes or until the buttercream is firm.

While your cake is chilling put your rainbow buttercream into piping bags with a small star nozzle and leave to one side. If you need to add your white buttercream to a piping bag with a large round nozzle.

Decorating with Rainbows

To make sure I got the perfect rainbow I used a rainbow cookie cutter and pressed it gently into the sides of my cake to get the imprint of the cutter on the cake. Repeat this all the way around the cake.

Now using your red buttercream pipe along the outline, the rainbow cookie cutter has given you.

Once you have piped the red buttercream continue piping with the orange, yellow, purple and turquoise.

Once you have piped all your rainbows following the outline of the cookie cutter using your white buttercream in a piping bag with a large round nozzle pipe some little round splodges on to represent the clouds. The cookie cutter I used had the outline for the clouds which made it slightly easier.

Decorating the top

So, you all know how I love to be a little bit OTT, so I decided to finish this cake off by piping the white buttercream over the top of the cake.

I finished it with some rainbow confetti sprinkles and a rainbow cake topper.

Alternatives

You can make this cake using any cookie cutters on the outside.

Also, you can use any colours for the rainbow you want.

Pin it For Later!

Print Recipe

5 from 7 votes