Preheat oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan. Prepare the chocolate cake mix according to package directions. Pour batter into the prepared pan and bake as directed, typically 28-33 minutes, or until a skewer inserted into the center comes out clean. Let the cake cool for 15-20 minutes.

While the cake is still warm, use the handle of a wooden spoon to poke holes all over the cake, about an inch apart. Gently warm the hot fudge sauce until pourable and pour it evenly over the cake, spreading it to fill the holes.

In a large bowl, beat the softened butter with an electric mixer until creamy. Add the marshmallow fluff and vanilla, and beat until combined. Gradually mix in the powdered sugar until fully incorporated, then beat on high for 2-3 minutes until the frosting is thick and fluffy.

Spread the marshmallow frosting over the cooled cake. For a toasted top, place the cake under the oven broiler for 30-60 seconds, watching constantly until the peaks are golden brown. Alternatively, use a kitchen torch.

Sprinkle with mini chocolate chips. Serve warm or at room temperature.