In a large bowl, mix together chocolate cake batter, with 1/4 water, 1/4 cup oil, and 2 eggs. Beat for 2 minutes. For the strawberry batter, mix together the vanilla cake mix, strawberry puree (just mash or blend fresh strawberries), 1/4 cup of water, 1/4 cup of oil, and 2 eggs. Blend for 2 minutes.
Using an ice cream scoop, spoon each flavor batter onto a cook sheet lined with parchment paper. Be sure to leave plenty of space in between. The batter will nearly double in size!
Bake at 350˚ for about 10-12 minutes of until firm. Make sure not to under cook them or they will become sticky and hard to handle.
Remove from oven and let cool for about 10-15 minutes.
While whoopie pies are cooling, fill your piping bag using a large star tip, with marshmallow fluff.
When the pies are cooled, take on into your hand, flat side up, and pipe marshmallow fluff around the edge and then fill in the center. Top your pie with the opposite flavor and you're done!
Store remaining cake mix for another batch, or double the recipe if you're feeding a crowd.