Prepare the Cupcake Batter: Combine the white cake mix, eggs, sour cream, milk, and vegetable oil in a mixing bowl. Mix for approximately 30 seconds, just until the ingredients are incorporated. Scrape down the sides of the bowl, then beat at medium-high speed for 3 minutes until the batter is well combined. Divide the Batter: Separate the batter evenly among 5 or 6 small bowls, depending on how many colors you plan to use. If using five colors, approximately 1 cup of batter per bowl should be sufficient.
Color Each Portion: Add food coloring to each bowl and mix until the desired shades are achieved. Gel food coloring is a useful choice because it produces vibrant colors without significantly thinning the thick batter.
Prepare the Cupcake Pan: Line a standard cupcake pan with paper liners. Keep a small bowl of water nearby to help spread the batter smoothly without it sticking to your fingers.
Layer the Colored Batter: Spoon the first color into each cupcake liner and gently spread it into an even layer using a finger dipped in water. Add the remaining colors one at a time, spreading each layer before adding the next. For five colors, approximately 2 teaspoons of each color per cupcake worked well in the original method. Adjust the amount to suit your liners and the number of colors, keeping the layers even. Bake the Cupcakes: Bake according to the directions on the white cake mix package. Check for doneness by inserting a toothpick into a cupcake; it should come out dry when the cake is ready.
Cool the Cupcakes: Allow the cupcakes to cool completely before applying the frosting. This helps prevent the buttercream from softening or melting against the warm cake.
Beat the Butter and Sugar: Place the room-temperature butter and powdered sugar in a mixing bowl. Beat until the mixture becomes light and fluffy.
Flavor the Frosting: Add the vanilla extract, coconut extract, and a dash of salt. Mix until the ingredients are evenly incorporated.
Adjust the Frosting Consistency: Gradually add the milk, beating as you go. You may not need all 5 tablespoons. Continue beating for 2–3 minutes, until the frosting is fluffy and suitable for spreading or piping.
Frost and Decorate: Spread or pipe the buttercream onto the completely cooled cupcakes. For a simple decorating option, spoon the frosting into a resealable plastic bag, snip off one corner, and pipe it onto the cupcakes. Finish with rainbow sprinkles if desired.
Create the Double-Rainbow Decoration: Reserve approximately 1/3 cup of white frosting and tint the remaining frosting blue. Frost the cupcakes with the blue buttercream. Place the reserved white frosting in a resealable bag and pipe cloud shapes on top. Cut an Airheads Xtremes candy strip in half and position it in the frosting clouds to create a rainbow arch, pressing the ends into the clouds so the candy stays upright.
