Preheat oven according to cake mix instructions. Grease a 9×13-inch baking pan
Prepare cake batter as directed on the box. Pour into pan and bake until a toothpick comes out clean
Let cake cool slightly while still warm. Using the wooden spoon handle, poke holes about 1 inch apart across the surface
Microwave 340 g chocolate chips and 14 oz sweetened condensed milk with 80 ml heavy cream in 30-second intervals, stirring after each, until smooth and glossy
Pour warm chocolate mixture over cake, letting it soak into holes. Spread excess with a spatula
Chill cake for 1 hour before adding marshmallow topping
In a mixing bowl, whip 360 ml heavy cream until thickened. Add powdered sugar and vanilla extract, continue whipping until stiff peaks form
Gently fold in 200 g marshmallow fluff until smooth and fluffy
Spread marshmallow mixture over chilled cake in even layers
In a microwave-safe bowl, combine 85 g chocolate chips and 45 ml heavy cream. Microwave in 30-second intervals until smooth, then refrigerate ganache, stirring occasionally until it thickens
Pipe or drizzle ganache across the top in decorative swirls
Chill completed cake for 1 hour before serving