How to Make a Fondant Rainbow Cake Decoration

How to make a rainbow cake decoration from marshmallow fondant.
A few months ago I shared the My Little Pony Cake I made for Aurelia’s third birthday. Sure it was adorable, but I had no idea it would become one of my most popular posts! Since then I’ve received quite a few e-mails asking me how I made the rainbow, and I was so glad to find that I had just enough marshmallow fondant leftover from Morgan’s Candy Shop Cake to finally make a tutorial.
Now since there was no real party to use this rainbow for, I had to make a cake specifically for this tutorial.  I simply baked an eight inch cake, chopped it in half to give me two “clouds”, and then slapped some frosting on top. Bam! Not my best work, but ehh, it’s two cakes. So let’s all just pretend that it is much prettier than it truly is and focus on the rainbow instead m’kay? Here we go…
How to make a rainbow cake decoration from marshmallow fondant.
 First, choose your cake plates, and determine the height difference that you like best. Yep. I’m totally going with a tealight holder and some salad plates, cuz I’m fancy like that. ;D You could easily make a rainbow that is level on both sides, but either way, it is important that you do this at least a few days in advance to give the fondant an adequate amount of time to dry. 
I find it easiest to bake my cakes, and wrap them tightly in plastic wrap once they’ve cooled. Then you can stack the layers to help determine the height you’ll need for the rainbow, and just pop them into the freezer until everything is ready to be assembled. This won’t affect the taste of the cake. Simply leave them set on the counter wrapped in the plastic until they are fully defrosted. Then you can unwrap and frost them as normal.
How to make a rainbow cake decoration from marshmallow fondant.
 Once you have your cakes stacked, hold a piece of paper behind both cakes, level with the top of the lower one. Draw a line to mark the top of the higher cake. This line will be one end of the rainbow, and the bottom edge of the paper will be the other.

How to make a rainbow cake decoration from marshmallow fondant.
Next, mark two vertical lines on each level to show approximately where you want the rainbow to touch the cake.
How to make a rainbow cake decoration from marshmallow fondant.
Connect the inner two vertical marks with a curved line, forming the underside of the rainbow. Then measure from this line out to the outside vertical marks. Adjust those marks so that both ends are the same width apart, and connect them with a second curved line. Make sure the two lines are the same width apart around the entire curve.
Make it easier: The width of your rainbow should be easily divided by the number of colors you plan to use. 
ex. For a 3 inch wide rainbow made with 6 colors, each stripe would be 1/2 in wide. Easy peasy.
How to make a rainbow cake decoration from marshmallow fondant.
Cut out your new template, and test it against the cake to make sure you are happy with the size and shape.
We’re done with the cakes at this point, so you can freeze them now if you plan to.

How to make a rainbow cake decoration from marshmallow fondant.
Roll out a bit of fondant, and use your template to cut out a base layer for the rainbow.
Don’t roll it too thin. It needs to be sturdy. (1/4 inch-ish)
How to make a rainbow cake decoration from marshmallow fondant.
Move your base to a stable surface like a cookie sheet where it will be able to rest safely while it drys. Yes, my sheet is gross, but I promise it’s clean! Press a few sturdy toothpicks into both lower edges of the base.  Press them into the fondant firmly, but be careful not to push them so hard that they come out the other side. This will help anchor the rainbow to the cake when you are ready to stand it up.
How to make a rainbow cake decoration from marshmallow fondant.
Roll out the color fondant you plan to use for the inner most stripe, and cut it into a long strip.
Use the width measurement from above divided by the number of colors to determine the width of you strips.

How to make a rainbow cake decoration from marshmallow fondant.
Apply a dab of water along the bottom edge of the base, and carefully glue the colored strip in place.
Be sure to use only a tiny drip of water, since too much will actually “melt” the fondant.

How to make a rainbow cake decoration from marshmallow fondant.
Carefully trim off the ends, and continue adding strips of the remaining colors side by side until you reach the top of the base. 
How to make a rainbow cake decoration from marshmallow fondant.
Set the pan aside, and allow the rainbow to dry. This could take a few days depending on the humidity in your area.
How to make a rainbow cake decoration from marshmallow fondant.

Once the rainbow has hardened simply frost your cakes as normal, and press the toothpicks down into the cakes.

Ta Da! A magical rainbow perfect for fairies, My Little Ponies, Care Bears, or whatever other cuteness you’ve got. Enjoy!

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*You might also be interested in the perfect Fluffy Cloud Icing 

to help decorate the ends of your rainbow. It’s adorable!*

12 thoughts on “How to Make a Fondant Rainbow Cake Decoration

    1. Thanks Samantha! After I got out of college I didn't want anything to do with fashion and thought I'd be a baker instead. Obviously I came back around to sewing in the end, but I guess a little of the cake decorating stuck with me. At least the kid's will have a few cute birthday cakes ;D

    1. Thanks Jenna! I've seen how creative you are though. You totally rock the furniture makeovers and decor, and trust me, I'm FAR from stellar at those! I have no doubts you could make one of these! ;D

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