Preheat oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan.

Prepare the cake mix according to the package instructions (using water, oil, and eggs).

Pour the batter into the prepared pan and bake for 30–35 minutes, or until a toothpick comes out clean.

While the cake is baking, heat heavy cream in a small saucepan until it just begins to simmer. Pour over the chocolate chips in a heatproof bowl. Stir until smooth and glossy, then stir in vanilla extract. Set aside.

Once the cake is done, remove it from the oven and let it cool for 10 minutes. Using the handle of a wooden spoon or a fork, poke holes all over the top of the cake.

In a small bowl, mix marshmallow cream with heavy cream until smooth. Spread the marshmallow mixture evenly over the cake, filling the holes.

Pour the chocolate topping over the cake and spread it evenly.

Let the cake cool completely before serving. Optionally, refrigerate for a couple of hours for a firmer texture.