Preheat oven to 400 degrees F (205 degrees C).
Line a baking sheet with parchment paper.
n a mixing bowl, combine the Red Velvet Cake Mix (1 box), Egg (3), Vegetable Oil (1/2 cup) and Water (1/2 cup) 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 (1 cup) until light and fluffy. Gradually add the Powdered Confectioners Sugar (4 cup). 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.