So of course my daughter found her birthday cake on Pinterest this year. Seems appropriate, right? She was looking over my shoulder one day and two months later, when I asked if she wanted anything in particular for her birthday cake, she didn’t hesitate. “The rainbow cake!”
I started researching recipes. I quickly found one from Whisk Kid, whose gorgeous cake got so much attention that she went on the Martha Stewart Show and BAKED. WITH. MARTHA. I figured if it was good enough for Martha, it would work for me.
I followed the cake recipe to the letter, and I would advise using a scale throughout the process. First, I actually weighed the ingredients that had weights provided in the recipe. And second, it was a very important step to weigh the batter at the end, and then divide that number by six, and weigh out the portions for each cake into individual bowls. This ensured that each layer was uniform.
It’s also very important to use gel food coloring, since you need their vibrance.
Here’s the link to the cake recipe. I made them a couple of days in advance, then wrapped each layer in plastic wrap and froze it.
Now, when it came time to think about the frosting, I had a problem. Fair warning: I am about to go all baker-nerdy. Feel free to skip this part. The original recipe called for a lemon-flavored Swiss buttercream, but my daughter prefers American buttercream and didn’t like the idea of a citrus flavoring. My favorite American buttercream forms a crust, but I wanted a buttercream that would not crust, because I thought it would be much easier to work with on a cake of this scale. So a friend of mine on Facebook who is an accomplished baker sent me her favorite, non-crusting American buttercream recipe, which I doubled to make sure I had enough for all six layers. If you make the doubled recipe (shown below), you should make it in two batches unless you have a large stand mixer (6 quarts).
Wendy’s American Buttercream (makes enough to frost and decorate the 6-layer rainbow cake)
3 cups butter, room temperature
1 1/3 cups crisco shortening
4 tsp clear vanilla extract
6-10 tbsp heavy cream (6 in hot and humid weather)
4 lbs icing sugar
Cream the butter and shortening in a stand mixer with the paddle attachment until there are no more lumps and it is creamy.
While the mixer is on a low setting add the vanilla and slowly add 6 tablespoons heavy cream. Mix well.
Gradually add the icing sugar, when it is all combined give it a good mix. Adjust the consistency, if it’s too stiff, by adding more cream a little bit at a time.
When it was time to assemble the cake, I fitted a large pastry bag with a coupler and loaded it with the buttercream. I got a 10″ cardboard round and set the first layer on the cardboard. I piped a rim of frosting around the outside of the cake, to make sure that the white layers of frosting would be consistent (you can see a good, quick visual that explains what I mean in this video).
Then, I repeated, making sure I was stacking the layers of cake as carefully as possible. Then, I did a crumb coat of frosting on the entire cake and put it in the freezer for about 15 minutes to set up.
Finally, I frosted it once more.
Obviously the cake would’ve been lovely if it had been all white on the outside with the riot of color inside, but the 8-year-old birthday girl wanted a little sizzle on the outside of the cake too. So, I made some rainbow buttercream by putting a dollop of frosting in each of 6 small bowls and then using the gel colors to shade them to match the cake layers. Then, I carefully spooned each color into a pastry bag, layering the colors in rainbow order.
Not sure if Martha would approve, but the kids loved it.