Instructions
Bake the cake according to the instructions on the box in a 9×13 pan.
Using the handle of a wooden spoon, poke holes all over the hot cake. Let cool slightly and prepare the filling.
Place the condensed milk, heavy cream and chocolate chips in a microwave safe bowl. Microwave for 30 seconds to 1 minute. Stir until smooth, microwave more, if necessary to melt the chocolate completely. The mixture should be pourable but not too runny. If it is too thick, reheat. If it is too thin, let it cool and thicken slightly. Transfer the mixture to a measuring jug and pour into the wells. Sprinkle the remaining chocolate over the top. Let cool, about an hour, then refrigerate until completely chilled.
To make the topping, whip the cream until it begins to thicken, add the powdered sugar and vanilla and whip until stiff peaks form. Gradually fold in the marshmallow fluff. Spread the frosting over the cooled cake.
Melt ½ cup chocolate chips with 3 tablespoons heavy cream until smooth. Transfer the mixture to a zip-top bag, cut off the corner and spread the chocolate ganache all over the top and use a toothpick or skewer to create a more marbled swirled pattern.