This rainbow drip cake is made with soft layers of funfetti cake, vanilla rainbow buttercream and white chocolate dripping down the sides.
Preheat oven to 350 °F. Spray three 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, 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. Add funfetti sprinkles and mix by hand.
Evenly distribute the cake batter among the three cake pans and bake for about 30-35 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 cakes in plastic wrap and place 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 5 minutes.
Turn the mixer to low, add vanilla, salt and gradually add the powdered sugar, followed by the cream. Mix until incorporated completely. Scrape down the sides of the bowl.
Turn the mixer to medium-high and beat the frosting for about 5 minutes until light and fluffy. Turn the mixer on low and mix for 2 minutes to push out any air bubbles.
Reserve half of the white buttercream to save for crumb coating. Divide the other half the white buttercream among six small bowls; color each bowl with gel food coloring. Make red, orange, yellow, green, blue, and purple colored buttercream.
Put each colored buttercream into six prepared piping bags with star piping tips.
In a medium glass bowl, melt white chocolate candy melts in the microwave by heating in 30-second increments, stirring in between until fully melted.
Mix in warm water to the melted chocolate with a rubber spatula. Mix until combined and streak-free.
Pour the chocolate into a squeeze bottle for decorating. If chocolate drip hardens then heat in the microwave for 5-10 seconds, shake or stir to make all the same temperature.
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.
Add a small amount of buttercream onto the center of the cake board. Layer the chilled funfetti cake, cut side up onto the cake board.
Pipe a ring of red buttercream in the center of the cake, followed by a ring of orange, yellow, green, blue and purple buttercream to the outside edge of the cake.
Place another cake layer cut side down over the top. Repeat the process for another layer — Crumb coat with the white buttercream and freeze for 30 minutes.
Then add a thicker layer of white buttercream and smooth the sides out using a cake scraper. Freeze for 30 minutes.
Drip the white chocolate drip over the sides and pour over the top, use an offset spatula to smooth out.
Then squeezing the red piping bag and moving your wrist into a circle you will create the to swirl. Start the next color buttercream on the middle of the previous swirl, repeat the process using all the prepared colored buttercream until the top of the cake is covered with the rainbow rope border.
Add the funfetti/rainbow sprinkles on the bottom of the cake. Serve and enjoy!