Preheat oven to 350°F (175°C) and prepare a 9x13-inch baking pan. In a bowl, combine the chocolate cake mix, eggs, water, and vegetable oil according to the package instructions. Mix until just combined and pour into the prepared pan. Bake for 25–30 minutes, until a toothpick inserted comes out clean. Allow to cool for about 10 minutes.

Once the cake has cooled slightly but is still warm, poke holes all over the surface using a large straw or skewer about 1 inch apart.

In a medium bowl, combine the sweetened condensed milk, heavy cream, and semi-sweet chocolate chips. Mix until smooth. Pour the filling mixture over the poked cake, letting it seep into the holes. Let sit for about 10 minutes.

In a bowl, whip the heavy cream until stiff peaks form. Gradually add the marshmallow fluff, folding it into the whipped cream until fully combined.

Spread the marshmallow whipped cream evenly over the cooled cake.

Melt the remaining semi-sweet chocolate chips with heavy cream in the microwave, stirring until smooth. Allow to cool slightly, then drizzle over the marshmallow topping.

Cover loosely with plastic wrap and refrigerate for at least 2 hours, or overnight for best results.