The biggest trick honestly through this process was finding a good recipe for frosting. Since the cake part was so easy I wanted to try and challenge myself. The best recipe I found out is called "butter-cream frosting" and finally after searching for what seemed like hours I ended up finding a great (and easy) one from Martha Stewart oddly enough, that I slightly altered. After a few tries I think I finally have us a winner:
(Following instructions from Betty Crocker cake box)
• included cake mix
• 3 large eggs
• 1/2 cup vegetable oil
• 2 sticks butter (softened)
• 3 cups powdered sugar
• 1/4 tsp vanilla extract
• 1/8 tsp salt
• preheat oven to 375°F
Follow the instructions from the back of the cake box, basically mixing the included mix, 3 eggs and vegetable oil together using an electric mixer. Using a spoon, place small amounts of batter on a parchment paper lined cookie sheet. (*This batter is definitely thick, the first time around I had difficulty using a spoon so the second time around I used a pastry bag and "piped" the batter making it much easier!)
Bake in the oven for 10-15 minutes or until you can poke it with a toothpick and the insides don't stick anymore.
While the whoopie pies are cooking and cooling, using the electric mixer beat the 2 sticks of butter until it forms a nice and creamy consistency. The begin adding powdered sugar 1/2 cup at a time mixing thoroughly. Add vanilla extract and salt and continue to mix.
Add small amounts of frosting to half of the whoopie pies and sandwich them to the other halves.
Now the best part, grab a whoopie pie, open your mouth and take a big bite! Hopefully you enjoy this delicious treat :)