Preheat oven to 350 degrees F. Grease a 9 x 13-inch baking pan with butter and set aside.
In a large bowl, combine cake mix,oil and eggs. Stir until well combined. Press half of dough into prepared pan.
In a small, microwave safe bowl, melt butter and milk chocolate chips; stir until smooth. Stir in sweetened condensed milk. Pour over crust and smoothing to edges. Drop remaining dough by teaspoonfuls over top.
Bake until edges are golden brown, about 30 minutes. Cool completely on wire rack before cutting into bars.
Adapted from Taste of Home, originally Yummy Cookie Bars