Turning 30 is definitely a milestone birthday. Maybe not physically but definitely mentally. So to celebrate turning 30 (back in October), I wanted to do something a little bigger and more memorable for my birthday dessert. This was definitely a more time consuming and elaborate production than the birthday cookie cake I made for my husband or the Cars birthday cake I made for my daughter, but it was big, beautiful, and absolutely delicious!
And where does anyone go these days when searching for inspiration? Pinterest, of course! I have zero experience working with fondant, so since I didn’t want my cake to be an unappetizing mess I decided to look for something memorable that didn’t include fondant. One idea possibility that kept popping up was for a rainbow layer cake, hidden beneath some unassuming buttercream frosting. It looked pretty straightforward but made a big statement, so I went with it.
A picture from Martha was all I needed to start to work. I knew I wouldn’t be needing to use or modify anyone’s recipe because I have absolutely no problem using boxed cake mix. It’s fast, it’s easy, and with two small kids running around it fits my needs perfectly.
How do you make a rainbow cake? Well if you’re looking for some birthday cake inspiration or something for your St. Patrick’s Day party that will really wow all of your family and friends, then I’d definitely urge you to give this cake a try.
Rainbow Layer Cake with Buttercream Frosting
2 boxes white cake mix
2/3 c. canola or vegetable oil
2.5 c. water
(6) disposable cake pans
Ingredients for Buttercream Frosting
1 c. unsalted butter (2 sticks), room temperature
4 Tbsp. milk
4 tsp. clear vanilla extract
4 c. powdered sugar
- The night before preparing the cakes and assembling, using a piece of wax paper and a pencil trace the outline of the bottom of your cake pan. You can use the disposable cake pans for tracing but the disposables I bought had a curved bottom so I used my own 9″ cake pan which had a uniform edge. From your template, you’re going to make 14 circles made of wax paper for this project.
- Cut out the circular wax paper pieces.
- Mix all buttercream frosting ingredients together until desired consistency. If your frosting ends up too thick, continue adding milk 1 tablespoon at a time until the mixture is what you’re looking for.
- Spread a thin layer of buttercream frosting on a wax paper cake pan template. Cover with a new circle and repeat, stacking your layers of buttercream frosting, until you have one wax circle left to place on top. You will have one extra layer of buttercream frosting in case any of your layers fall apart during assembly.
- Carefully place your dividing layers of buttercream in the freezer until ready to assemble your cake the following day.
- Refrigerate remainder of buttercream frosting until ready to frost cake.
- Set aside remaining wax circles until the following day when you’ll coat with cooking spray and use these to line the bottom of your disposable baking pans. Lightly spray additional cooking spray to sides of cake pans.
- The next day, preheat your oven according to cake mix box directions – usually 350 degrees.
- Mix cake mix, eggs, oil, and water together until blended (about 2 minutes).
- Evenly divide cake batter in mixing bowls.
- Add gel food coloring to each individual bowl, adding a small amount at a time until you achieve your desired colors.
- Pour batter into baking pans and place all pans in oven to cook for approximately 8-12 minutes. Make sure to keep an eye on your cakes during this stage as cooking times will vary and these cakes are very thin.
- Once cakes are done, allow to cool in pan for 10 minutes before inverting pans on baking racks to finish cooling.
- When the cakes are cooled, if needed using a serrated knife cut tops to make flat edges for the layers.
- On a cake stand or foil covered surface, place your bottom layer of cake followed by a frozen layer of buttercream frosting. Continue alternating cake and frosting until all layers are assembled. Since this cake is so heavy, I cut three straws to the height of the cake and stuck them into the cake to provide stability.
- Icing a cake this size can be really tricky. I used a plastic spatula to plop some frosting on top of the cake, then I used a metal offset spatula to smooth it all over creating a thin layer to seal in the crumbs. Then repeat using more icing and spread an even layer of buttercream frosting on your cake. You can use the edge of your metal spatula to smooth things out. If you’re looking for an even smoother, more finished look I’ve heard a bench scraper would really make a difference and get the job done perfectly.
The white frosting on this cake is really unassuming and is sure to surprise those around when it’s served and the beautiful rainbow is exposed beneath. To add a little more color to the outside, you could always decorate it with sprinkles, candies, or other colors of icing.