Preheat oven to 350. Prepare a 13" x 9" pan with cooking spray and flour; set aside.In a mixing bowl, combine cake mix, oil ,and egg. Mix until crumbly. Reserve one cup crumb mixture. Press remainder onto bottom of prepared pan. Meanwhile, in a small saucepan, combine sweetened condensed milk, chocolate chips, vanilla extract, and salt. Over medium heat, cook and stir until chips have melted. Pour evenly over prepared crust. Sprinkle reserved crumb mixture evenly over top. Bake 25 minutes, or until bubbly.