Preheat oven to 350°F. Line baking sheets with parchment paper.
In a large bowl, mix the cake mix, eggs, oil, and a few drops of yellow gel food coloring if using. The food coloring is optional but helps create a brighter yellow cookie. Fold in the sprinkles.
Scoop large dough balls, about 2 tablespoons each, onto the prepared baking sheets.
Bake for 9-11 minutes or until the edges are set and the centers look slightly underbaked.
Cool on the baking sheet for 5 minutes, then transfer to a cooling rack and cool completely.
For the frosting, beat the butter until smooth and fluffy.
Gradually add the powdered sugar, then mix in the vanilla and enough cream to create a thick, pipeable buttercream.
Tint the frosting bright Toy Box blue with gel food coloring.
Pipe or spread a generous layer of frosting onto the flat side of half the cookies.
Top with the remaining cookies and gently press together.
Roll the exposed frosting edges in the red, yellow, and blue sprinkles.
Chill for 15 to 20 minutes before serving if you’d like a firmer filling.