Soft rainbow cake layers filled with creamy vanilla buttercream and topped with a fondant rainbow that has a golden candy surprise in the center.
On a silicone fondant mat, knead and roll out a piece of white fondant, then cut a straight line for the base of the rainbow arch. Then cut a small circle on the bottom of the white fondant. Keep the circle cutter there to be the anchor so you can layer the rainbow colors on top of it.
Next, roll each rainbow color into a long rope starting with purple, attach it to the white fondant with a little bit of water. Then repeat with blue, green, yellow, orange, and red.
Cut the white fondant to be the same size as the rainbow and trim the rainbow ropes to be a straight line.
Then cut the 20-gauge wire into two pieces then insert the wire into each side of the rainbow. Set aside to dry.
Preheat oven to 350 °F. Spray three 6-inch round cake pans (or six 6-inch round cake pans) with non-stick spray, line the bottom of each pan with parchment paper, and spray again. Set aside.
In a medium-size bowl, whisk and stir together the cake mix, all-purpose flour, and salt. Set aside.
In the stand mixer bowl fitted with a paddle attachment, beat the butter, oil, and sugar on medium-high for 5 minutes.
Add eggs, one at a time to the stand mixer bowl and beat on medium until well incorporated.
Turn the mixer on low and add half of the flour mixture to the butter mixture until fully incorporated.
Add the vanilla and milk and beat on low until incorporated. Scrape down the sides of the bowl. Finish by adding the remaining flour mixture and mix on medium for another 30 seconds.
Evenly distribute the cake batter among the six mixing bowls, for 6-inch cake pans it's 3 ½ large ice cream scoops per bowl. Color each mixing bowl with one of the following rainbow colors, super red, orange, lemon yellow, leaf green, sky blue, and violet.
Then pour the colored cake batter into three cake pans to bake in two parts (or six cake pans and bake all at one time) and bake for about 15-20 minutes or until a toothpick inserted into the center of each cake comes out clean or with just a few moist crumbs.
Remove cakes from the oven and let cool in pans for 10 minutes before inverting onto cooling racks to cool completely. Wrap the six rainbow cake layers in plastic wrap and place it into the freezer to chill for 30 minutes.
In the stand mixer bowl fitted with the paddle attachment, beat the butter until light and fluffy, about 2 minutes.
Turn the mixer to low and add salt and vanilla.
Next, gradually add powdered sugar, followed by the cream. Mix on medium-high speed for about 5 minutes, until light and fluffy. Scrape down the sides of the bowl.
Before using turn the mixer on low and mix for 2 minutes to push out any air bubbles or use a rubber spatula to push out the air bubbles. (You may need to repeat this step when frosting the outside of the cake).
Fill a prepared piping bag fitted with a #844 star piping tip and fill halfway with buttercream. Set aside for later.
Remove the cake layers from the freezer and unwrap the cake layers. Next, use a serrated knife to cut off the dome of the cakes to make the layers flat and even.
Cut a circle with a circle cutter in the center of the orange, yellow, green, and blue cake layers.
Add a small amount of buttercream onto the center of the cake board. Layer the purple cake, cut side up onto cake board. Add vanilla buttercream over the cake using an offset spatula to spread buttercream and make it even. Then add the blue cake on top, add a thin layer of buttercream, try not to cover the center hole with buttercream, it's best to leave it unfrosted.
Continue with the green, yellow, and orange cake layers make sure the center hole lining up correctly. Add the gold candy and sprinkles into the center hole. Then add buttercream on the orange cake layer.
Place the red cake layer cut side down over the top. Repeat the process for another layer — Crumb coat with vanilla buttercream and freeze for 30 minutes.
Add a thicker layer of buttercream to the cold cake. Use an offset spatula and cake scraper to make the sides as smooth as possible.
Then place the cake onto a baking tray on top of the turntable. Add rainbow sprinkles to the bottom of the cake. The baking tray will catch any loose sprinkles.
Pipe a rope border with the star tip along the top edge of the cake. Start by squeezing the piping bag and move your arm in a counterclockwise motion, continue the circular motion around the cake until the rope border is completed.
Insert the rainbow fondant into the top of the cake, so it's standing up. Finally, pipe swirls with the star tip on either side of the rainbow to cover the wires and make it look like clouds.
Serve at room temperature and enjoy the golden candy surprise!