Preheat the oven according to the cake mix directions.

Grease a 9×13-inch pan and prepare the batter as directed on the box.

Bake until a toothpick inserted in the center comes out clean.

Cool the cake for 5 minutes while it is still warm.

Use the handle of a wooden spoon to poke holes evenly over the top of the warm cake, about 1 inch apart.

In a microwave-safe bowl, combine sweetened condensed milk, 1/3 cup heavy cream, and 12 oz semi-sweet chocolate chips.

Microwave for 30–60 seconds, then stir until smooth.

Pour the chocolate mixture slowly into the holes, letting it soak through the cake.

Spread any remaining mixture over the top.

Allow the cake to cool for 1 hour at room temperature, then refrigerate for 2–3 hours until fully chilled.

In a large bowl, whip 1 1/2 cups heavy cream until soft peaks form.

Add powdered sugar and vanilla; continue whipping until stiff peaks form.

Gently fold in the marshmallow fluff until smooth.

Spread the marshmallow whipped cream evenly over the chilled cake.

In a small bowl, melt 1/2 cup chocolate chips with 3 tablespoons heavy cream until smooth.

Transfer to a piping or zip-lock bag and drizzle over the top. Swirl with a toothpick for a marbled look.

Chill for 30 minutes before serving.

Slice and enjoy cold.