1-Bake cake according to box directions in: Bake cake according to box directions in a 9×13 cake pan.
2-Using the handle of a wooden spoon,: Using the handle of a wooden spoon, poke holes all over the warm cake. Set aside to cool slightly and make the filling.
3-Place sweetened condensed milk, heavy cream, and: Place sweetened condensed milk, heavy cream, and chocolate chips in a microwave-safe bowl. Microwave for about 30 seconds to 1 minute. Stir until smooth, microwaving more if needed to melt the chocolate completely. The mixture should be pourable but not too runny. If it’s too thick, reheat it. If it’s too thin, set aside to cool and thick slightly. Transfer the mixture to a measuring jug and pour in the holes. Spread the remaining chocolate on top. Set aside to cool, for about an hour, then refrigerate until completely cooled.
4-To make the topping, whip heavy cream: To make the topping, whip heavy cream until it begins to thicken, add powdered sugar, vanilla, and whip until stiff peaks form. Gradually mix in marshmallow fluff. Spread the topping over the cooled cake.
5-Melt 1/2 cup chocolate chips with 3: Melt 1/2 cup chocolate chips with 3 Tablespoons heavy cream until smooth. Transfer the mixture to a zip-lock bag, and cut off the corner. Pipe the chocolate ganache all over the top and use a toothpick or skewer to create a more marbleized pattern of swirls.