Preheat your oven to 350°F (175°C). Prepare the chocolate cake mix according to the package directions for a 9×13-inch pan. Bake as directed, or until a toothpick inserted into the center comes out clean.

Let the cake cool in the pan for about 10-15 minutes. While it’s still warm, use the round handle of a wooden spoon to poke holes all over the cake, spacing them about an inch apart.

In a large microwave-safe bowl, combine the sweetened condensed milk, 1/3 cup of heavy cream, and 12 oz of chocolate chips. Microwave in 30-second intervals, stirring well after each, until the chocolate is completely melted and the mixture is smooth and pourable. If it becomes too thick to pour easily, reheat it gently for a few seconds.

Pour the chocolate mixture evenly over the warm cake, making sure to fill all the holes. Use a spatula to spread any remaining mixture smoothly over the top. Let the cake cool completely on a wire rack, then refrigerate for at least 2 hours, or until the filling is set.

In a large, chilled mixing bowl, use an electric mixer to whip the 1 1/2 cups of heavy cream until it begins to thicken. Add the powdered sugar and vanilla extract, and continue whipping until stiff peaks form. Gently fold in the marshmallow fluff by hand using a spatula until just combined and no white streaks remain.

Spread the marshmallow whipped cream topping evenly over the chilled, filled cake.

For the ganache, combine the remaining 1/2 cup of chocolate chips and 3 tablespoons of heavy cream in a small microwave-safe bowl. Microwave in 20-second bursts, stirring until smooth. Let it cool slightly for a minute or two. Transfer the ganache to a small zip-top bag, snip off a tiny corner, and pipe lines or a drizzle pattern over the top of the frosting. Use a toothpick or a knife to gently swirl the ganache into the frosting to create a marbleized effect.

Refrigerate the finished cake for at least another hour to set the swirls. For the best flavor and texture, let the cake sit at room temperature for about 15-20 minutes before slicing and serving.