Preheat oven to 350 degrees. Grease a 9x13" baking dish.
Mix cake mix, oil, eggs and water on low for 30 seconds.Mix on medium speed for 2 minutes.
Pour batter into cake pan. Bake for 28-30 minutes or until toothpick inserted comes out clean. Let cake cool completely.
Marshmallow Filling
Beat butter, powdered sugar and vanilla together until whipped and creamy. Stir in marshmallow creme by hand.
Spread filling over cooled cake with a rubber spatula or butter knife. Chill the cake for 30-45 minutes.
Chocolate Ganache
Place chocolate chips into a bowl.
Heat the heavy cream in a small saucepan on the stove until it begins to steam and bubble around the edges (not boil). Remove from heat.
Pour the hot cream over the chocolate chips. Let stand 3-5 minutes. Stir chocolate and cream until completely smooth.
Pour ganache over cake and spread evenly. Chill cake for 30-45 minutes, just until the top is set. Allow cake to come to room temperature then cut into slices.
Store leftover cake at room temperature, covered in plastic for up to 5 days. This recipe makes 12 giant slices or 24 regular slices of cake.
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Notes
Calories for each serving of food and nutrient information based off of box of Duncan Hines dark chocolate fudge cake mix.