What’s at the end of a rainbow? Cake, of course! Make this bright and cheery 3-D easy rainbow cake for a birthday celebration, baby shower or just because! Add a handful of chocolate gold coins around the fluffy buttercream clouds and it becomes the perfect ending to a St. Patricks’s Day supper.
You don’t need a degree in engineering to make this 3D rainbow cake stand up. It’s easy-peasy, with no carving or special tools required!
How to make a 3-D easy rainbow cake
- 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 & cut the cake
Level your cake by trimming off the domed top. Normally, you’d slice a cake in half horizontally to create layers. For this 3-D cake, cut the cake in half vertically to create two semi-circles. Eyeball it, or use a ruler to find the center of the cake before cutting it.
Step 2: Stack & coat the semi-circle cakes
Top one of the cakes with a layer of buttercream and place the other semicircle of cake on top. Crumb coat the cake with a thin layer of plain buttercream. Pop the cake in the fridge until the filing is firm, about 15 minutes.
Step 3: Stand the cake up
Spread a thin layer of buttercream onto the flat side of the cake. Stand the cake 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. Pop the cake 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 is in the fridge, trace the outline of the cake pan onto a piece of parchment or waxed paper. Cut the circle out and then in half.
Draw 5 arching lines on the semi-circle of paper, creating a rainbow. I freehanded the lines, but you could use a compass to be more precise.
Cut along the longest line to remove the outermost strip.
Step 5: Tint your buttercream
Divide your buttercream into six portions. Tint each portion 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, greased side down, and press the paper against the buttercream. Use a small offset icing spatula to spread pink buttercream over the exposed arc of cake.
Step 6: Onto the next color
Peel the paper off of the cake. Cut along the longest line to remove the outermost strip. Place the paper back on the cake, greased side down.
Use a small offset icing spatula to spread orange buttercream below the pink buttercream. Peel the paper away and repeat the cutting and spreading process until all of the rainbow colors have been applied. Pop the cake back into the fridge if the buttercream starts to soften.
Step 7: Pipe the clouds
Fill a piping bag with the remaining white buttercream. Snip the tip of the bag and pipe white fluffy clouds around the base of the rainbow. Sprinkle the clouds with white nonpareils for added texture.
Get Started Decorating!
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