Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan. Mix Devils Food Chocolate Cake Mix according to package instructions and pour into the pan. Bake for 25-30 minutes or until a toothpick comes out clean. Allow to cool for 10 minutes.

Gently poke holes about an inch apart across the surface of the warm cake using a wooden spoon handle. This will help the ganache seep into the cake.

In a microwave-safe bowl, combine sweetened condensed milk, semi-sweet chocolate chips, and heavy cream. Heat for 30-45 seconds, stirring every 15 seconds until smooth. Pour the ganache into the holes and spread any extra over the top of the cake. Refrigerate for at least 30 minutes.

In a mixing bowl, add heavy cream and powdered sugar. Whip on high until stiff peaks form, about 3-5 minutes. Fold in marshmallow fluff to create the topping.

Melt additional semi-sweet chocolate chips with a splash of heavy cream in the microwave, stirring until smooth. Drizzle over the whipped cream topping.

Refrigerate the completed cake for at least 1 hour before serving. Slice into squares and serve chilled.