Preheat your oven to 350°F (175°C). Prepare the boxed chocolate cake mix according to package instructions. Pour the batter into a 9x13 inch baking pan and bake for about 30-35 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool completely in the pan.

Once the cake cools, poke holes all over the top of the cake, about 1 inch apart. This will create channels for the ganache to seep into.

In a saucepan, heat 1/3 cup of heavy cream over medium heat until it simmers. Remove from heat, add chocolate chips, and whisk until smooth. Pour evenly over the poked cake.

In a mixing bowl, combine 1 1/2 cups of heavy cream and powdered sugar. Whip until stiff peaks form, then fold in marshmallow fluff.

Spread the marshmallow whipped cream over the ganache-covered cake. Optionally, swirl additional ganache on top for decoration.

Refrigerate for at least 1 hour before serving. Slice into squares and serve with fresh strawberries or vanilla ice cream.