Preheat your oven according to the package instructions for the Devil's Food Chocolate Cake Mix. Grease a 9x13-inch baking pan and prepare the cake batter as directed on the box.

Pour the prepared batter into the greased pan and bake until a toothpick comes out clean, about 25-30 minutes. Cool in the pan for about 5 minutes.

Poke holes in the top of the warm cake using the handle of a wooden spoon, spaced about 1 inch apart and going three-quarters of the way down.

In a microwave-safe bowl, combine the sweetened condensed milk, 1/3 cup of heavy cream, and semi-sweet chocolate chips. Microwave for 30-60 seconds, stirring until smooth.

Pour the ganache over the poked cake, ensuring it seeps into all the holes. Let the cake cool to room temperature for about 1 hour.

Transfer the cooled cake to the refrigerator and chill for at least 2-3 hours.

Whip 1½ cups of heavy cream until soft peaks form. Gradually add powdered sugar and vanilla extract, whipping until stiff peaks form. Fold in marshmallow fluff until smooth.

Spread the marshmallow topping evenly over the cake, covering it completely.

Melt the remaining chocolate chips with 3 tablespoons of heavy cream until smooth. Drizzle over the marshmallow topping and swirl with a toothpick.

Return the cake to the refrigerator for an additional 30 minutes to set before serving.