Preheat your oven to 350 degrees.
Combine the cake mix with the vegetable oil and eggs. Spray a 9x13 baking dish with non stick cooking spray. Spread HALF of the cake mix mixture evenly into baking dish. I find that using a silicone spatula sprayed with nonstick cooking spray really helps with this.
In a saucepan over medium heat, combine the sweetened condensed milk with the chocolate chips & butter. Stir until melted and smooth, about 2-4 minutes .
Pour the chocolate mixture over the cake mix that's in the casserole dish.
Dollop the remaining cake batter mixture evenly over the chocolate fudge. Using a knife, pull the knife through the cake from one end to another making swirls. Bake at 350 degrees for 20 minutes.
