For the Rainbow Unicorn Cake

First, make the red, orange, and yellow cake layers:

• Prep your oven and pans. Position a rack in the center of the oven and preheat the oven to 350°F. Generously spray three 8-inch round cake pans with cooking spray and line the bottom of each with a parchment paper circle. Spray the parchment, too.
• Make the cake batter. Place the Pillsbury Funfetti cake mix in a large bowl. Use a tall glass to press down into the center of the mix to make a "well".
• Pour the water, oil, and egg whites into the well and mix with a rubber spatula until just combined. At this point, your batter will likely still be lumpy—that's okay, I promise!
• Set a small bowl on a digital scale and tare it to “0”. Pour batter into the pan until the scale registers 11 ounces (312 grams). Repeat with a second and third bowl. If necessary, redistribute any leftover batter evenly between the three bowls. Each bowl should contain between 11 ounces (312 grams) to 11.15 ounces (316 grams) of batter.
• Add yellow food coloring, either with a ¼ teaspoon measure or 1 to 2 generous drops at a time, to one bowl of cake batter. Mix with a small rubber spatula or spoon until completely combined. Keep adding food coloring until the cake batter is bright yellow and very saturated (but be sure not to add more than 1 teaspoon of food coloring—see baker's notes for more info). Once done, pour the batter into one of the prepared cake pans.
• Add orange food coloring, either with a ¼ teaspoon measure or 1 to 2 generous drops at a time, to a second bowl of cake batter. Mix with a small rubber spatula or spoon until completely combined. Keep adding food coloring (however, no more than 1 teaspoon of food coloring) until the cake batter is bright orange and very saturated. Once done, pour the batter into one of the prepared cake pans.
• Add red food coloring, either with a ¼ teaspoon measure or 1 to 2 generous drops at a time, to the last bowl of cake batter. Mix with a small rubber spatula or spoon until completely combined. Keep adding food coloring (however, no more than 1 teaspoon of food coloring) until the cake batter is bright red and very saturated. Once done, pour the batter into the remaining prepared cake pans.
• Bake the cakes. Bake for 16 to 18 minutes, or until the top of each cake bounces back when gently pressed. A skewer inserted into the center of the cake should come out with a few crumbs attached. Cool the cakes in their pans on a wire rack for 20 minutes, then turn out onto the wire rack and cool completely. The cakes can be stored unfrosted, wrapped tightly in plastic wrap, at room temperature for up to 2 days and in the freezer for up to 3 months.

Next, make the green, blue, and violet cake layers:

• Prep your oven and pans. Position a rack in the center of the oven and preheat the oven to 350°F. Generously spray three 8-inch round cake pans with cooking spray and line the bottom of each with a parchment paper circle. Spray the parchment, too.
• Make the cake batter. Place the Pillsbury Funfetti cake mix in a large bowl. Use a tall glass to press down into the center of the mix to make a "well".
• Pour the water, oil, and egg whites into the well and mix with a rubber spatula until just combined. At this point, your batter will likely still be lumpy—that's okay, I promise!
• Set a small bowl on a digital scale and tare it to “0”. Pour batter into the pan until the scale registers 11 ounces (312 grams). Repeat with a second and third bowl. If necessary, redistribute any leftover batter evenly between the three bowls. Each bowl should contain between 11 ounces (312 grams) to 11.15 ounces (316 grams) of batter.
• Add blue food coloring, either with a ¼ teaspoon measure or 1 to 2 generous drops at a time, to one bowl of cake batter. Mix with a small rubber spatula or spoon until completely combined. Keep adding food coloring until the cake batter is bright blue and very saturated. Once done, pour the batter into one of the prepared cake pans.
• Add purple food coloring, either with a ¼ teaspoon measure or 1 to 2 generous drops at a time, to a second bowl of cake batter. Mix with a small rubber spatula or spoon until completely combined. Keep adding food coloring (however, no more than 1 teaspoon of food coloring) until the cake batter is bright purple and very saturated. Once done, pour the batter into one of the prepared cake pans.
• Add green food coloring, either with a ¼ teaspoon measure or 1 to 2 generous drops at a time, to the last bowl of cake batter. Mix with a small rubber spatula or spoon until completely combined. Keep adding food coloring (however, no more than 1 teaspoon of food coloring) until the cake batter is bright green and very saturated. Once done, pour the batter into the remaining prepared cake pans.
• Bake the cakes. Bake for 16 to 18 minutes, or until the top of each cake bounces back when gently pressed. A skewer inserted into the center of the cake should come out with a few crumbs attached. Cool the cakes in their pans on a wire rack for 20 minutes, then turn out onto the wire rack and cool completely. The cakes can be stored unfrosted, wrapped tightly in plastic wrap, at room temperature for up to 2 days and in the freezer for up to 3 months.

Now, make the first batch of frosting to assemble and crumb coat the cake:

• Make the frosting. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, vanilla, and salt on medium-low until smooth and creamy, about 1 minute. Increase the mixer to high and beat until light and fluffy, 2 to 3 minutes, using a rubber spatula to scrape down the bottom and sides of the bowl as necessary.
• Reduce the mixer to low and add the confectioners' sugar 1 cup at a time, beating until combined. Scrape down the bottom and sides of the bowl with a rubber spatula. If the frosting seems too stiff, add 1 to 2 teaspoons of the milk and beat on low just to incorporate the liquid. Scrape down the bottom and sides of the bowl once more, then beat on high until the frosting is creamy and smooth, 2 to 3 minutes.

Next, assemble and crumb coat the cake:

• Place the purple cake layer on a cake board or serving platter in the center of a rotating cake stand. Use a rubber spatula or cookie dough scoop to drop ½ cup of frosting in the middle of the cake. Spread it evenly all over the top, just like you would spread butter on toast. Place the blue cake layer on top of the frosting, stacking it evenly on top of the first cake. Drop another ½ cup of frosting in the middle of the cake and spread it evenly. Repeat this process of positioning the layers and frosting them in this order: green, yellow, orange, and red on top.
• Cover the surface of the entire layer cake with a thin layer of frosting. Use just enough frosting to cover the entire cake completely. Refrigerate, uncovered, for 30 minutes to stiffen and "set" the frosting.

While the crumb coat sets, make the second batch of frosting:

• Make the frosting. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, vanilla, and salt on medium-low until smooth and creamy, about 1 minute. Increase the mixer to high and beat until light and fluffy, 2 to 3 minutes, using a rubber spatula to scrape down the bottom and sides of the bowl as necessary.
• Reduce the mixer to low and add the confectioners' sugar 1 cup at a time, beating until combined. Scrape down the bottom and sides of the bowl with a rubber spatula. If the frosting seems too stiff, add 1 to 2 teaspoons of the milk and beat on low just to incorporate the liquid. Scrape down the bottom and sides of the bowl once more, then beat on high until the frosting is creamy and smooth, 2 to 3 minutes.

Then, finish frosting the cake:

• Remove the cake from the refrigerator and place it back in the center of the rotating cake stand. Cover the cake completely with the remaining frosting, using an offset spatula to spread the frosting all over the top and sides of the cake evenly. Once the cake is completely covered, hold the length of the offset spatula's blade against the frosting as you rotate the cake stand to smoothen the frosting. Once the frosting is perfectly smooth, refrigerate the cake while you prepare to make the unicorn cake's buttercream mane.
• Divide the remaining frosting between three small bowls. Add 1 to 2 small drops of blue food coloring to one bowl of frosting. Mix with a small rubber spatula or spoon until completely combined. The frosting should be a pale, pastel blue. Once done, transfer to a piping bag fitted with an open star tip.
• Add 1 to 2 small drops of red or pink food coloring to a second bowl of frosting. Mix with a small rubber spatula or spoon until completely combined. The frosting should be a pale, pastel pink. Once done, transfer to a piping bag fitted with a small closed star tip.
• Add 1 to 2 small drops of purple food coloring to the final bowl of frosting. Mix with a small rubber spatula or spoon until completely combined. The frosting should be a pale, pastel lavender. Once done, transfer to a piping bag fitted with a closed star tip.
• Remove the cake from the refrigerator and place it back in the center of the rotating cake stand. Arrange the unicorn horn and ears on top of the cake. Use the different piping bags to pipe little dollops of frosting on top of the cake and down one side of the cake in a cascading fashion. Keep one side of the cake completely free of frosting—this is the the unicorn cake’s face! Carefully place the unicorn eye stickers on the side of the cake with no frosting to create a unicorn face.
• Serve and store. Serve immediately. The assembled unicorn rainbow cake can be stored at room temperature, under a cake dome or a large bowl turned upside down, for up to 1 day. Press a sheet of plastic wrap against any cut surfaces to prevent the cake from drying out. After that, cover the entire cake loosely with plastic wrap and refrigerate for up to 2 days.