Whoopie Cake

Preheat oven to 350 degrees F. Butter two 8 ” round baking pans and line each with parchment paper. Butter the bottom and sides of the pans and dust with flour. Combine cake mix, water, melted butter and eggs in a medium bowl. Whisk using a hand mixer until smooth. Divide batter between pans. Bake for 15-20 minutes, until toothpick inserted in center comes out clean. Cool for 10 minutes in the pans, then invert cakes onto cooling racks to cool completely. While the cake is baking, make filling and ganache. To make filling, combine softened butter, confectioners' sugar, fluff, vanilla and salt in the bowl of a stand mixer. Beat until smooth and fluffy. To make ganache, place chocolate chips in a medium, heatproof bowl and set aside. Heat heavy cream in a small saucepan over medium heat. When bubbles begin to break the surface around the edges of the pan, turn off the heat. Pour the hot cream over chocolate chips, whisking constantly until the sauce is smooth. Top one round cake with marshmallow frosting, then place the second cake on top of it. Pour ganache over the cake.