Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan. Prepare the chocolate cake mix according to the package instructions, mixing until smooth. Pour the batter into the prepared pan and bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean. Allow the cake to cool in the pan for about 10 minutes before moving on to the next step.

Once the cake has cooled slightly, use the handle of a wooden spoon to gently poke holes throughout the cake, spacing them about 1 inch apart.

In a small saucepan over low heat, combine the sweetened condensed milk and semi-sweet chocolate chips. Stir continuously until the mixture is smooth and all the chocolate has melted, about 5 minutes. Pour the ganache evenly over the poked cake.

In a large mixing bowl, add the heavy cream and powdered sugar. Using an electric mixer, whip the cream on medium speed until stiff peaks form, about 3–5 minutes.

Gently fold the marshmallow fluff into the whipped cream until fully combined. Spread this mixture evenly over the cooled cake.

Drizzle any remaining chocolate ganache over the top of the marshmallow whipped cream. Chill the cake in the refrigerator for at least 1 hour before serving.