1Heat oven to 350°F (325°F for dark or nonstick pan). Lightly grease or spray bottom and sides of 13x9-inch pan.
2In medium bowl, mix butter and cake mix with fork until crumbly; reserve 1 cup. Press remaining mixture in bottom of pan.
3Bake 10 minutes. Sprinkle chocolate chips over baked layer. Drop frosting by tablespoonfuls over chocolate chips. Stir milk into reserved cake mixture. Drop by teaspoonfuls onto frosting layer.
4Bake 24 to 28 minutes or until cake portion is slightly dry to touch; cool 10 minutes. Carefully loosen edges of bars with knife. Cool completely. Refrigerate until firm. For bars, cut into 8 rows by 6 rows. Store loosely covered.