Preheat oven to 350 degrees. Line a baking sheet with parchment paper and set aside. If you using a whoopie pie pan like me, coat with butter or shortening and dust generously with flour. Tap out excess flour.
In a large mixing bowl, add in the cake mix, eggs, water, and oil. Beat with a hand mixer on medium until incorporated.
Once combined, fold in sprinkles.
Using a ¼ measuring cup, drop batter onto a lined baking sheet about 1 to 2 inches apart. If using the whoopie pie pan, pop it in each floured cavity.
Bake for 12-15 minutes. Allow to cool for 5 minutes and turn out onto a wire rack to cool completely.
In a medium bowl, add butter, powdered sugar and 1 tablespoon of milk. Beat with hand mixer on low. If the frosting is too thick, add an extra tablespoon of milk. Once powdered sugar is incorporated, beat on medium/high for 30 seconds to a minute.
Fold in sprinkles.
On the flat side of one whoopie pie, spread center filling and sandwich with another whoopie pie.