Rainbow Cake for Dummies

Disclaimer: I am not an advanced baker. There are 7000 other posts on how to do a rainbow cake or a rainbow unicorn cake or an animated dancing rainbow unicorn cake on the internet if you’re beyond this level. However, if you’re more on the cake-from-a-box-with-simple-frosting kinda gal and want to take it one little notch up, this post is for YOU! It’s fairly detailed with tips and tricks since I needed them at one point or another.

My oldest was due on December 24th and through some crazy antics I managed to get myself into labor a few days early so she wouldn’t be an actual Christmas baby. However, her birthday still falls within the time that allllll the Christmas decorations, songs and festiveness are in full effect. This makes me endeavor to have an extra-special NON-HOLIDAY related birthday celebration for her to calm my inner mom-guilt. The last couple of celebrations, I bought fancy cakes for her out of laziness but this year we’re on a more strict budget so I decided to DIY it up.

Now, I may not be a fancy baker but I am SUPER picky about how my desserts taste. I let her pick the flavor of cake and frosting. She boldly and without hesitation declared she needed chocolate cake and rainbow frosting (anyone else try to do light colored frosting on chocolate cake before and gotten that lovely cookies-and-cream crumb look? Luckily I’ve already made that mistake and knew how to problem solve but I knew it would be a tedious process to say the least). I tried to convince her that funfetti frosting is sort-of-rainbowish, but she didn’t buy it. Here’s how I pulled it off!

Step 1: Cake

I got chocolate cake mix – my homemade cakes turn out delicious but flat despite years of problem solving so I made like Elsa and Let It Go. I love the Trader Joes one. I always, always make the cakes with melted butter. They taste a million times better and vegetable oil is realllllllly bad for you. Yes worse than butter. You can also use coconut oil if you can’t do dairy. I made a double batch the day before, baked them in 3 pans instead of 4 to have tall layers and let them cool completely – this is KEY STRATEGY #1. The chocolate cake will fall apart into your frosting if it’s not alllllll the way cool and set. Also put the butter out to soften overnight for frosting.


Step 2: Frosting

I’ve played around with lots of different frostings but in the end my all time favorite is a classic buttercream. Make more than you think you need because you do not want to have to skimp! If you forgot to soften butter, you can microwave on 10% or lowest power for 30 second intervals until soft. DO NOT get impatient with this. 10%. 30 second intervals. Trust me.

Simple Buttercream (for a double batch)

  • 1 lb (4 sticks) unsalted butter (go organic if you can), softened
  • tiny pinch salt
  • 4-6 C or one bag powdered sugar
  • 2-4 Tbs milk (option to use lemon juice, liquor or other flavored liquid if desired, can sub in 1 tsp vanilla or almond extract in as well)
  • Optional: food coloring

Cream butter in mixer for 1-2 minutes until lightened. Scrape down bowl. Sprinkle salt around and sift in 1 C sugar at a time. Beat slowly until incorporated then faster until homogenous. Add in 1 Tbs liquid at a time if the frosting starts to get too stiff. Alternate with sugar until sweetness and texture is as desired. Will take at least 4 Cups.

Step 3: Color

I planned out my rainbow to be pastel based on the food coloring we had. I needed a big bunch of purple for the top and purple for between the layers since that was her favorite color and then less of each other color for the sides.

  • Purple: I separated out about 2/3 of the white frosting and added the purple dye (or use pink/red + blue) a few drops at a time, scraping down and beating in frequent alternations until the right color got there. Don’t over-beat your frosting or it will separate.
  • I then divided up the remaining 2/3 white frosting into 5 roughly even portions and did the same process to make pink (or you could do red), orange, yellow, green and blue. Place each color into one corner of it’s own plastic baggie and set aside. I did this with the children. I had 0 teaspoons of patience left at the end of this process so sent them away with their dad for step 4.
  • If it’s really hot in your kitchen, your frosting might be too soft – refrigerate for 15 minute intervals until it spreads like soft butter.

Step 4: Assembly

  1. Check your alignment: Since I overfilled the cake pans (I’m gonna call it on purpose…), one of them was lop sided so I sliced it off flat.
  2. Put down a TBS of whatever frosting you have the most of in the middle of your chosen cake plate and place first layer of cake crumblier side down – for me this was the cut side. Use strips of wax paper to protect the edges of your plate. Cutting strips of wax paper can be a good task for a 4-7 year old kid.
  3. Get a tall glass or shallow wide bowl of very warm water and set aside your frosting-wiping rag
  4. Put some large dollops of frosting around the middle on top of the cake. If your frosting spreads easily, spread with short strokes from middle of cake out to edges. If it’s sticky or stiff at all, warm your metal spreader in the water, dry off and then spread. Spread right to the edge, but not over the side.
  5. Place second layer on top and repeat step 4 for the top of cake
  6. Start the sides with the bottom color. Use toothpicks to mark out where each color goes if it helps you. I just eyeballed it. Cut about 1cm off the corner of the baggie with your first frosting color. Squeeze in a line around the bottom with one hand as you spin the cake plate with the other. Don’t worry about it being perfect, just get a line on there. Repeat going up the side with the other colors. Go back at the end and fill in any larger gaps with the appropriate color.
  7. Take your frosting utensil, warm in the warm water, wipe clean and again, using one hand while the other spins the plate, run it vertically around the side of the cake to smooth and smoosh the colors together. Smooth the top into a corner. 

  8. Add sprinkles if you feel extra-fancy and VOILA!

Chill the cake until ready to serve. Let warm to room temperature about an hour before enjoying. Cut with warm knife for optimal aesthetically pleasing slices.



Dr. Annie is an experienced family doctor, pretty-ok baker and mom of 2 with 1 more coming very soon in California.

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Real As A M*ther

Four girls became best friends in high school and have stayed together through a whole lot of life. We are now a doctor, a lawyer, a financial advisor and a badass doula slash massage therapist and homesteader and want to share what we've learned as wives, moms, women and in our careers with the world... and entertain you along the way!

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