The good news: This standout (and stand up!) cake is way easier to make than it looks, and no carving tools are needed. The bad news: Actually, there is none. We're here to make all your rainbow dreams come true with a cake full of awesome, and it's the perfect centerpiece for a birthday bash, baby shower or whatever else comes your way.
- 10" round cake
- 4 cups of vanilla buttercream
- Red, orange, yellow, green, blue and purple gel food coloring
- Small offset spatula
- Parchment or waxed paper
- Piping bag
- White nonpareils
Step 1: Level and cut the cake
Level your cake by trimming off the domed top. Now usually you'd slice a cake in half horizontally to create layers, but for this one we'll just cut the cake in half vertically to create two semi-circles. Feel empowered to eyeball it, or use a ruler if you want to be exact.
Step 2: Stack and coat the semi-circle cakes
Top one of the cakes with a layer of buttercream and place the other semicircle on top. Crumb coat the cake with a thin layer of plain buttercream, then pop it in the fridge until the filing is firm (about 15 minutes).
Step 3: Stand the cake up
Next up, spread a thin layer of buttercream onto the flat side of the cake and stand it up, using that layer of buttercream to adhere it to the plate or cake board. Finish the cake with a thicker crumb coat of white buttercream and put it back in the fridge until the icing is firm to the touch (at least 10 minutes).
Step 4: Make your rainbow template
While the cake's chilling, trace the outline of the cake pan onto a piece of parchment or waxed paper.
Cut out the circle, then cut it in half.
Draw 5 arching lines on the semi-circle of paper to create the rainbow. I went freehand here, but you could also use a compass if you're stressing.
Cut along the longest line to remove the outermost strip.
Step 5: Tint your buttercream
Reserve some buttercream for the clouds you'll pipe at the end, but divide the remaining buttercream into six bowls and tint each a different color of the rainbow using your gel colors.
Step 6: Add the first color
Flip the semicircle over and lightly grease the back of the paper with shortening. Line the paper up with the cake, with the greased side against the cake, and press it to the buttercream. Use a small offset icing spatula to spread your first color for the top arc of the cake.
Step 6: Onto the next color
Peel the paper off of the cake, then cut along the next longest line to remove that strip. Put the paper back on the cake, greased side down.
Use a small offset icing spatula to spread orange buttercream below the pink. Peel the paper away and repeat the cutting and spreading process until all the rainbow colors have been applied. If the buttercream starts to soften while you work, just pop the cake back in the fridge to firm it up.
Step 7: Pipe the clouds
Fill a piping bag with the reserved white buttercream. Then snip the tip of the bag and pipe white fluffy clouds around the base of the rainbow. Sprinkle the clouds with white nonpareils if you're feeling really fancy.