Preheat oven to 350 degrees. For the Cake: Grease and flour a 12-cup bundt pan; set aside. In a large bowl, combine the dry cake mix, dry pudding mix, water, oil, eggs, sour cream and vanilla. Using a hand mixer, mix until all ingredients are completely incorporated and mixture is smooth. Pour batter into the prepared bundt pan and spread out evenly. It is a very thick batter. Bake for 45 minutes or until a toothpick inserted in the center comes out clean. Remove from oven and cool in pan for 10 minutes, then invert to a wire rack and cool completely. Once cake is cool, slice in half horizontally.
For the Marshmallow Creme Filling: In a large bowl, using a hand mixer, mix butter until smooth. Add the marshmallow creme, salt and vanilla and mix until well combined. Add one cup of powdered sugar and mix until combined. Add remaining one cup of powdered sugar and mix until smooth and creamy. Spread filling over the bottom half of the cake and place top half of cake over the creme filling.
For the Chocolate Ganache: Place chocolate chips in a small bowl; set aside. Microwave whipping cream in a small, microwave-safe, dish until it starts to bubble. Pour hot cream over chocolate chips and allow to sit for 5 minutes. Stir together until smooth and creamy. If chocolate chips aren't completely melted. Place in the microwave and heat, stirring every 10 seconds until mixture is smooth and chocolate is melted. Allow to cool for 10 minutes or until slightly thickened and pour over cake. Store cake in the refrigerator. Bring to room temperature before serving.