I’ve been reminded for weeks that someone wanted “a rainbow cake with rainbow icing” for his birthday cake. Ok. No problem. My first thought was to use it as an opportunity to try a mirror glaze! Unfortunately, I realized I wouldn’t have the time in the week leading up to the birthday so I went through several other ideas – 6” rounds each of a different color, two 9” rounds with the colors carefully layered – before I settled on a tie-dye 9”x13” and decided to wing it with the design of the buttercream.
For the cake, I used a boxed white cake mix and the recipe on the box. I substituted milk for water and melted butter for oil. Note that you can either us just egg whites or the entire egg with the box recipes. Since I wasn’t leaving any of the cake white, I used the whole egg. After making the cake batter, I separated it into five individual bowls. I used Betty Crocker gel coloring for each color except for purple, which was a liquid coloring. I realized too late into coloring that my dark colors had gone bad so I was not going to get the typical red, yellow, green, etc. The colors ended up very bright which ended up being perfect! I alternated pouring batter of different colors into the greased 9×13 pan so they sat on top of each other. Half of each bowl would get poured in. After each half, the second half was added to keep the variety going in the layers. When the batter was all in the pan, I realized it looked like the imaginary food the kids had a food fight with in the movie “Hook”! Now I’m calling this bake “Imagination Cake” (imagination pie was referenced in the food fight scene). Take a toothpick and run it up and down the length of the cake or make swirl designs to add additional twists of color. Place in the oven for 30 mins at 325F. When done, let cool.
For the frosting, I went with I Am Baker’s whipped vanilla buttercream.
2 sticks of butter, softened
2 tsp vanilla extract
2 tsp almond extract
4 cups of confectioner’s sugar, sifted (her recipe goes as high as 8 but I found the minimum to be perfect)
2 Tbsp whole milk (She notes that you can go up to half a cup for a more creamy and loose icing. I found 2 Tbsp to be perfect.)
1 pinch of salt
Beat butter in the bowl of a stand mixer with he whisk attachment on medium-high speed until light and fluffy – about 3 mins. Add vanilla and almond extracts. Continue to mix.
With mixer on low, slowly add the sugar, milk, and salt, frequently scraping the sides and bottom of the bowl. Once incorporated, whip the frosting for at least 3 minutes on medium to high speed. I ran mine for 7 minutes, same as in the recipe.
If the frosting is too thick to spread, gradually beat in the additional milk (up to a half cup, total).
Separate into five different bowls. Add selected food dye to each bowl and stir until combined. Place an open baggie into a small cup with the top of the bag folded over the lip of the cup. Pour frosting from one bowl into one bag. Lift up bag, zip or twist to close, and use scissors to snip a small tip off of one of the bottoms of the bag. Holding the bag over the cooled cake, squeeze the bag to pipe the icing in a straight line longways along the cake. Use a butter knife, rubber spatula, or icing knife, smooth the row of colored frosting across the cake and down the sides. Repeat the bagging and popping steps for your next color. Overlapping color in each row helps with coverage. Repeat until the entire cake is covered. Run a toothpick through the frosting, lengthwise, to give the frosting a little extra flair. I ended up with plenty of leftover frosting but I also didn’t make a thick layer of frosting on the cake. Let sit or place in the fridge to firm up. It’s been so hot here that I popped mine in the fridge overnight. It gave the frosting a nice texture. The frosting also softened up again nicely after sitting out for a bit.
The almond extract works really well with the other flavors and makes this frosting stand out.