Preheat oven to 350º and spray a 9x13 pan with cooking spray.
Beat the cake mix, eggs & oil with an electric mixer until blended. Press 2/3 of the mixture into prepared pan. Microwave the sweetened condensed milk, chocolate chips and butter on high for 1 minute until smooth. Pour over crust. Crumble rest of crust mixture evenly over the top. Bake for 20-25 minutes until lightly browned. Cool and cut into bars.