The Rainbow Cactus Birthday Cake


This is one of those projects that always gets a, “Wait, what?” when I bring it up in conversation. (Pro tip – bringing up a rainbow cactus cake is a great way to liven up a dull topic). Luckily for me, the back story on this one does actually kind of make sense.

I’ve been making rainbow birthday cakes for my sister for several years now, using  this excellent post from the Whisk Kid blog. So, of course, for her birthday last year this cake was a given. But she had an additional request – could I make it into a cactus? (She’s very into cacti. And also flamingos, but that’s not currently relevant.) I was pretty sure she was joking, although it’s a bit hard to tell, but who cared? I never turn down a challenge, so a cactus cake it would be – with a rainbow surprise inside.


I played around with the idea of making a cake in the shape of the iconic Saguaro cactus, but decided against it. (I’m going to claim that that was just to retain some sanity, but it was really because I desperately wanted to make chocolate frosting). Instead, I settled on a round cactus in a pot. I planned to make a rectangular cake and a round cake, and then carve the round cake into a hemisphere once it was all assembled.

Fair warning: This cake is insane. Even a regular rainbow cake, with its six individual layers, is a full afternoon and evening project – this cake took me several days. I added to the already complex task by using several different fillings and frostings. You could simplify it by sticking with one kind of frosting and no filling, or just one filling. You could also make a non-rainbow cactus cake, and simplify the baking process. But, if you’re crazy like me (or just want to steal part of this), here’s the step-by-step process.

1. A day or two in advance, make cactus spines. (Apparently I forgot to take pictures of this). Make a bowl of royal icing (I use this recipe – you can make royal icing with raw eggs, but it’s a bit iffy, and meringue powder gives much better results) and, using a number 1 or 2 tip, pipe long, thin lines onto parchment paper. After they have dried for a day or two, use a pizza wheel or sharp knife to cut them into pieces of about the right size. They will break – don’t worry about it.



2. Using the directions from the Whisk Kid Post, bake your six layers. I doubled the recipe, if my memory serves me correctly. I baked my layers in a 9 inch round cake pan and in a 9×5 loaf pan. To get super bright colors, I used AmeriColor soft gel food coloring, which I highly recommend if you work with colors a lot – you can get much purer, brighter colors and a lot more variety. However, you could probably manage this project with standard grocery store coloring without too much trouble.



3. Make whichever fillings and frostings your heart desires. In the picture above, clockwise from top left, are what I used: Peanut butter frosting, Italian meringue buttercream, chocolate frosting, cream cheese frosting, Smucker’s salted caramel sauce, raspberry jam, and lemon curd. I’m not going to go into detail on any of these right now – maybe in future posts – but there are plenty of great tutorials available online on any of these that you choose. The fillings really are a matter of personal preference. I used the chocolate, peanut butter, and caramel for the plant pot, and the others for the cactus itself.



4. Stack and fill your cake. I stacked my circles into a tall cylinder, then, using a sharp knife, carefully carved away until I had a hemisphere. There’s no trick to this, unfortunately – take it slow, turn it often, and don’t be afraid to fill in holes with pieces that you have already cut off. I promise that no one will notice.

Once you’re got the shape you want, stick the two bits together and crumb coat the whole thing with a thin layer of frosting. Then put it in the fridge for a few hours, minimum – I left mine overnight – to let it set.



4. Decorate! I frosted the whole thing in an additional layer of chocolate (pot) and green-colored Italian meringue buttercream (cactus). Then I tinted both frostings to be darker and piped lines. I used a star tip to make swirls of dark green frosting, then stuck individual cactus spines in by hand. (This was a serious pain and takes a while – be prepared).

Then all that’s left to do is stick in some candles, sing “Happy Birthday”, and work out how on Earth you’re going to eat this behemoth of a cake.

Should you make this cake, or a variation of it, I would love to see it! Leave a comment or use the contact link to show it off.


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Millennial math teacher with a crafting problem. I've never met a new skill that I didn't want to learn - everything from cactus cakes to paper flowers, dying yarn to sewing dresses, churning butter to making shoes. (Still working on that last one). My adventures are sometimes successful, often wacky, and always challenging. Stick around for step-by-steps, helpful resources, book reviews and exciting narration, suitable for beginners and experts alike.

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