Rainbow Prism Cake

My husband and daughter recently sang in the Cambridge Science Festival Chorus‘s production of

Songs of Electromagnetic Radiation

so it seemed only fitting that I should make a cake to match the theme for the post-performance party.

Rainbow Prism Cake

Rainbow Prism Cake

The party guests were wowed by the candy glass prism on top of the cake, and they all rushed to take pictures of it, and to take pictures of themselves standing next to it.

They didn’t realize the surprise that was in store for them once it was cut.


Cut Rainbow Prism Cake, revealing the rainbow layers inside.

Molding the Prism

The first task was to make a candy glass prism.

It’s possible to make candy glass using table sugar, but sugar caramelizes at a lower temperature than the one necessary to make the glass, so it has a slightly golden hue rather than being crystal clear.  For the bright, vivid colors of a rainbow encased in a clear prism, using isomalt was the answer.

Thankfully, once it’s initially made, isomalt glass can be reheated several times for reuse, so I made a single batch of it and tinted small amounts in rainbow colors, using Americolor gel colorings.

Isomalt in rainbow colors

A lovely array of rainbow-colored isomalt.

I had made molds using matboard (the kind used in framing pictures), rolled teflon, and teflon mouse tape.  The molds consisted of two triangular faces for the prism, three rectangular sides, and multiple slats that could be added or removed sequentially to create the stripes of the internal and external rainbows.

Prism Rainbow Internal

Internal Prism Rainbow

Prism Rainbow External

External Prism Rainbow

I heated up each color and poured, then moved on to the next when it has solidified enough.

After pouring all six colors, it was clear to see that they were really separate segments stuck together, but I was able to smooth them out with careful use of a culinary blowtorch, a flat block of wood (protected by teflon, of course), and a weight.

Prism Rainbow

Pretty rainbow!

Assembling the Prism

I assembled the prism by melting the edges with a combination of a culinary blowtorch and using the surface of a cast iron griddle.  It’s delicate, fussy, and somewhat dangerous work (I only got one small sugar burn on a finger), but worth it.  One of the sides was broken during assembly, but I decided we could do without the bottom.

Assembled Rainbow Prism

Rainbow Prism, sans input light beam.

Assembling the Cake

Initially I thought I would make all of the layers from scratch, but the baking aisle at the supermarket tempted me with six perfectly colored cake mixes for a dollar apiece, so they came home with me.  Each mix made a single 8″ layer, and I torted them to approximately 1/3 of their original size (the rest go into the freezer, because surely there will be a future need for rainbow cake?).

I filled between the layers with Italian meringue buttercream, plus a hidden layer of blackberry jam between the blue and violet tiers…since some people feel that dropping indigo from the spectrum is not acceptable.

Rainbow Cake Layers

Rainbow Cake Layers

I iced the outside with the same buttercream, hastily added ridges to the sides, and piped a border.  The candy glass topper was added immediately before serving, so that the moisture from the icing wouldn’t damage it.

Rainbow Cake Frosted

Rainbow Cake, Frosted

Voila!  Rainbow Prism Cake to wow the geeky masses (and wow them it did).

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8 Responses to Rainbow Prism Cake

  1. Pingback: Cookie Microwave Background | Geeky Cakes

  2. Rose Winter says:

    What was the recipe you used for the italian buttercream? Was that also vegan or did it use aquafaba for the eggs?


    • lindajulien says:

      This particular cake was not vegan, but I have made vegan Italian Meringue Buttercream. It can indeed be done with aqua faba! I need to make another round of it and write up a blog post.


      • Rose Winter says:

        Thanks so much for your response, Linda! Please do write it up! I want to try to make that kind of frosting, but was wondering how the aquafaba would hold up with the fat (dairy or non-dairy butter) being added. Also, not sure if Swiss or Italian buttercream recipes would work better over another… thoughts?


    • lindajulien says:

      I was able to make vegan IMBC using my regular recipe by substituting aqua faba for the egg whites (1:1 by volume), and 100% palm oil shortening for the butter (1:1 by weight). The end result is fluffier and lighter in texture than conventional IMBC, and it lacks the depth of flavor that you would normally associate with dairy butter, but the texture is fantastic. I am hoping to find a way to firm it up a bit (perhaps by reducing the aqua faba first?) so that it can be used successfully under fondant…I’m not sure that my current recipe *won’t* work under fondant, mind you, but it feels softer and less firm than the conventional stuff, so I worry.

      Other have reported making Swiss Meringue Buttercream, but I have not tried it myself.

      You’ve inspired me to dig up the notes I’d made to start this blog post, so I hope you’ll see one on the topic soon!


      • Rose Winter says:

        So happy to encourage you to find the recipe! I can’t wait to read about it… and follow in your footsteps!


  3. NicoleK says:

    Indigo is totally a fake spectrum color. You did well to make six layers! 😉

    Liked by 1 person

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