Preheat an oven to 350°F (180°C). Coat six one-eighth sheet pans (each 6 by 9 inches/15 by 23 cm) with nonstick cooking spray.

In the bowl of an electric mixer fitted with the flat beater, cream the butter on medium speed until light and fluffy, about 2 minutes. Stop the mixer and scrape down the sides of the bowl. With the mixer on medium speed, add the eggs one at a time, beating well after each addition and scraping down the sides of the bowl as needed. Reduce the mixer speed to low and add the cake mix in three additions, alternating with the milk and beginning and ending with the cake mix. Increase the mixer speed to medium-high and beat until the batter is smooth, about 1 minute.

Divide the batter evenly among 6 bowls; there will be about 3/4 cup or 7 1/4 oz. (205 g), of batter in each bowl.
For the pink layer, add 1/8 tsp. of pink food colouring to one bowl of batter. Stir until the colour is uniform and no streaks of white remain. Repeat to colour the orange, yellow, green, blue and purple layers, following the table below for the food colouring amounts:
Pink 1/8 teaspoon.
Orange 1/8 teaspoon.
Yellow 1/8 teaspoon.
Green 1/8 teaspoon.
Blue 1/8 teaspoon.
Purple 1/4 teaspoon.

Pour each bowl of coloured batter into a separate prepared pan. (If you don’t have 6 sheet pans, you can bake the layers in batches.) The pans will be less than half full. In batches if necessary, bake the cakes, rotating the pans once during the baking time, until a toothpick inserted in the cakes comes out clean, about 8 minutes. Transfer the cakes to wire racks and let cool in the pans for 10 minutes, then turn the cakes out on the wire racks to cool completely.

Using a 1 3/4-inch (4.5-cm) biscuit cutter, cut out 12 rounds from each cake. You will need to cut the rounds very close to each other to yield 12. Set aside.

In the bowl of an electric mixer fitted with the flat beater, cream the butter and cream cheese together on medium-high speed until smooth, about 1 minute. Reduce the mixer speed to low and slowly add the frosting mix. When all the frosting mix has been incorporated, increase the mixer speed to medium-high and beat until light and fluffy, about 2 minutes. Transfer the frosting to the piping bag (included) and, using scissors, cut a 1/2-inch (12-mm) opening at the tip of the bag.

Place the sprinkles in a small bowl and set aside.
Working with one push pop container at a time, take the top off the container and use the plunger to push the base all the way to the top. Place a purple cake round on the base, then pipe a small dollop of frosting (about 1 teaspoon) in the centre of the cake round. Place a blue cake round on top of the purple round and press down to adhere, then pipe another dollop of frosting on top of the blue round. Repeat with a green, yellow, orange and pink cake round in that order, piping a teaspoon of frosting between each layer. Finish by piping a teaspoon of frosting on top of the final (pink) layer.
Dip the finished cake pop in the bowl of sprinkles to coat the final layer of frosting with sprinkles. Place the top on the push pop container and repeat with the remaining containers to assemble the remaining cake push pops. Serve immediately or store in the refrigerator for up to 3 days.
Makes 12 cake push pops.

Flour Shop x Williams Sonoma Rainbow Pop-Up Cake Kit