Preheat oven to 400 degrees. Line a baking sheet with parchment paper.
In a mixing bowl, combine the cake mix, eggs, oil and water with a hand mixer. Beat for about 3 minutes until the batter is smooth and creamy.
Using a spoon or scoop, measure even amounts of batter onto the parchment paper. Place them a couple of inches apart to allow for expansion during cooking.
Bake for 7-9 minutes. Remove from the oven and let cool for 5 minutes. Transfer to a wire rack and cool completely. (Repeat the process until all of the batter has been used. Use a new sheet of parchment paper with each batch.)
While the cakes are baking, make the buttercream frosting. On high speed using a hand held mixer, cream the butter until light and fluffy. Gradually add the powdered sugar. Continue to mix for 5 minutes until smooth and fluffy.
Spoon the frosting into a decorating bag. Snip the end off, and swirl a layer of icing on the inside of a cake. Place another cake on top.
Decorate the top of each whoopie pie as desired.
Serving size and nutritional value is based on a total of 12 pies about 4 inches large.