Prepare the chocolate cake in a 9 × 13 inch baking pan, following the instructions on the cake mix package. Once the cake comes out of the oven and is still warm, use the handle of a wooden spoon to poke holes evenly across the surface. Let the cake cool for a few minutes while you prepare the chocolate filling.

In a microwave-safe bowl, combine the sweetened condensed milk, heavy cream, and chocolate chips. Heat the mixture in the microwave for 30 second intervals, stirring after each, until the chocolate has completely melted and the filling is silky smooth. It should flow easily from a spoon without being overly thin. If it feels too thick, warm it for a few more seconds. If it becomes too loose, allow it to cool for a few minutes until it thickens slightly.

Pour the warm chocolate mixture into a measuring jug, then slowly drizzle it over the cake, making sure it fills the holes. Spread any remaining chocolate filling evenly over the top. Leave the cake at room temperature for about an hour, then refrigerate until thoroughly chilled.

For the marshmallow topping, whip the heavy cream until it begins to hold its shape. Add the powdered sugar and vanilla extract, then continue beating until stiff peaks form. Gently fold in the marshmallow fluff until the mixture is smooth and fluffy. Spread the topping evenly over the cold cake.

To finish, combine the chocolate chips and heavy cream in a microwave-safe bowl and heat until smooth, stirring well to create a glossy ganache. Spoon the ganache into a small zip-top bag and snip a tiny corner. Pipe thin lines across the marshmallow topping, then drag a toothpick or skewer through the chocolate to create elegant swirls with a marbled finish.