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Day 2: Make the frosting and build the cake
Beat the butter: Beat the butter in the bowl of a stand mixer fitted with a paddle attachment on medium speed until smooth.
Gradually add in the sugar: Turn the mixer speed to low and slowly add in the 7 cups of the sugar. Once it's all added in, increase the speed to medium-low and mix until thoroughly combined.
Add the liquid ingredients: Reduce the mixer to low, add in the cream or milk and vanilla, and mix until combined.
Whip until creamy: Once everything is incorporated, mix on medium speed until smooth and creamy.
Add the remaining sugar: Reduce the mixer speed to low and add in the remaining 1/2 to 1 cup of sugar until the buttercream is as firm as you want it. Turn the mixer back up to medium and whip again until fluffy.
Designate your cake layers: Stack the still-wrapped cakes and decide what order your rainbow will be. Designate which cakes will be the middle two layers. For our purposes, purple and blue will make the foundation of the cake, green and yellow the center, and orange and red the top two layers.
Build the foundation of the cake: Put a dab of frosting in the center of a cake or serving plate. Line the cake plate with 4 strips of parchment paper to keep the cake plate clean while you frost. Unwrap the purple cake layer and remove its parchment paper. If needed, use a long, serrated knife to level the cake. Place the leveled cake on the center of the cake plate. Top with 3/4 cup of frosting and smooth into an even layer. Some crumbs may stick into the frosting, but don't worry — you won't see them once the cake is complete. Unwrap the blue layer, remove the parchment, and level the cake. Invert the blue layer on top of the purple layer, making sure to align the two cakes as much as possible. Top with 3/4 cup of frosting and smooth into an even layer.
Make the filled layers: Unwrap the green layer, keep the parchment on, and level if needed. Use a 3-inch-wide round cutter to cut a hole in the center of the green cake layer and use an offset spatula to remove the small round of cake. Use the green cake's parchment paper to easily invert the green cake over the blue layer and align the cakes as much as possible. Remove the parchment paper. Frost the green ring with 1/2 cup of frosting, keeping clear of the hole. Repeat unwrapping, leveling cutting a hole, and placing yellow cake on top of the green cake. Remove the parchment and frost with 3/4 cup frosting, this time also frosting the interior of the cake's hole with a thin layer of frosting.
Fill the cake with sprinkles: Pour the sprinkles into the cavity in the green and yellow layers. Press the sprinkles in the sides and bottom of the cavity, being sure the center is quite full and packing in the sprinkles.
Make the final layers: Unwrap the orange layer, remove its parchment, and level if needed. Place the orange layer on top of the yellow cake layer and align with the other layers. Top with 3/4 cup of frosting and smooth into an even layer. Unwrap the orange layer, remove its parchment, and level if needed.
Invert the red layer on top of the orange layer and align the cakes as much as possible.
Crumb coat the cake and chill: Place 1/2 of the remaining frosting on top of the cake and use an offset spatula to spread it into a thin layer on the top and sides of the cake. Some crumbs will be picked by the frosting, but don't worry — you'll over those with another thin layer of frosting in a bit. Refrigerate the crumb-coated cake for 1 hour. Cover the remaining frosting with plastic wrap and set aside at room temperature.
Frost and decorate the cake: Top the chilled cake with the remaining frosting and spread into a thin, even layer on the top and sides of the cake. Top with any additional sprinkles, if desired.
Cut and serve the cake: Refrigerate the cake for at least 1 hour more or even overnight before slicing and serving. Use a long, sharp knife to slice through the finished cake, and be prepared that the tip of the knife will meet some resistance as it goes through the sprinkles.
Recipe Notes
Make ahead: Bake the cake layers and store them at room temperature for up to 1 day.
Storage: The assembled cake can be stored in the refrigerator for up to 5 days.

Wow! Doesn't that sound amazing? This is definitely an ideal cake to make for a children's birthday party, although foodies of all ages will think it's delicious. You might want to make two, because everyone's going to want a second slice!

Article Source: The Kitchn

http://www.thekitchn.com/how-to-make-the-ultimate-rainbow-surprise-cake-242554