In a bowl, combine cake mix, eggs and oil. With floured hands, press two-thirds of the mixture into a greased 13 x 9 baking pan. Set remaining cake mixture aside.
In a microwave-safe bowl, combine the sweetened condensed milk, chocolate chips, peanut butter and butter. Microwave, uncovered on high for 45 seconds, stir. Microwave 45 to 60 seconds longer or until melted. Stir until smooth, pour over crust. Drop teaspoonfuls of remaining cake mixture over top.
Bake at 350º for 20 to 25 minutes or until lightly browned.