Good news: All your rainbow dreams are about to come true with this colorful, tasty cake. Even better news: It’s way easier to make than it looks, and no carving tools are needed. Consider it the perfect centerpiece for a birthday bash, baby shower or St. Patrick’s Day party.
What You Need
1. Level and Cut
Trim off the domed top of your cake with a cake leveler. Cut the cake in half vertically to create two semi-circles — you can either eyeball it or use a ruler for a perfectly even split.
2. Stack and Coat
Top one of the cakes with a layer of buttercream, then 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).
3. Stand It Up
Spread a thin layer of buttercream onto the flat bottom of the cake and stand it up, using that layer of buttercream to adhere it to the plate or cake board. Add a thicker crumb coat of buttercream, then put it back in the fridge until the icing is firm to the touch (at least 10 minutes).
4. Make Your Rainbow
While the cake chills, 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 five arching lines on the semi-circle of paper to create the rainbow. We drew freehand, but you could also use a compass for exact lines.
Cut along the longest line to remove the outermost strip.
5. Tint the Buttercream
Reserve some buttercream for clouds; set aside. Divide the remaining buttercream into six bowls and tint each a different color of the rainbow using your gel colors.
6. Add the First Color
Flip the semicircle over and lightly grease the back of the paper with shortening. Line the paper up grease-side against the cake and gently press into the buttercream.
Use a small offset icing spatula to spread your red color for the top arc of the cake.
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 are applied. If the buttercream softens while you work, place the cake back in the fridge to firm up.
7. Pipe the Clouds
Fill a piping bag with the reserved white buttercream. Snip the tip of the bag and pipe white fluffy clouds around the base of the rainbow. Sprinkle the clouds with white sugar pearls.