Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan and prepare the boxed chocolate cake mix according to the instructions on the package. Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Allow the cake to cool completely in the pan.

Once the cake has cooled, poke holes evenly throughout the cake, about 1 inch apart. The holes should extend down almost to the bottom for maximum ganache absorption.

In a microwave-safe bowl, combine sweetened condensed milk and semi-sweet chocolate chips. Heat in the microwave for 30-second intervals, stirring until the chocolate is melted and smooth. Carefully drizzle the ganache filling into each of the holes.

In a large mixing bowl, pour in chilled heavy cream and add powdered sugar. Beat on medium-high speed until stiff peaks form, about 3-5 minutes. Gently fold in marshmallow fluff.

Spread the fluffy marshmallow whipped cream mixture evenly over the top of the cake using an offset spatula. For an added touch, drizzle any remaining melted chocolate ganache over the whipped topping.

Cover the cake with plastic wrap and refrigerate for at least 30 minutes to allow flavors to meld together. Slice and enjoy your delicious cake!