Preheat oven to 350°F. Reserve 1 cup cake mix and 1/3 cup evaporated milk to use in Filling.
For BASE: Beat remaining cake mix, butter, remaining evaporated milk and walnuts in a large mixing bowl at low speed of an electric mixer just until combined. Press into a greased 13 x 9-inch glass baking dish. (TIP: To prevent mixture from sticking to fingers, use a greased sheet of wax paper or greased spatula to press evenly into dish.)
Bake 10 minutes. While base is baking, prepare FILLING.
For FILLING: Combine reserved 1 cup cake mix, reserved 1/3 cup evaporated milk, corn syrup, sugar and eggs in a medium mixing bowl. Beat 3 minutes at medium speed of an electric mixer; beat in vanilla.
Sprinkle semi-sweet and white chocolate chips over partially baked base. Carefully pour filling mixture over base.
Bake 25 to 30 minutes or until cake is lightly browned around edges and just set. Cake may still appear slightly soft in center. Cool on a wire rack. Cut into squares. (TIP: Use a greased plastic knife for easy cutting.)
TIP: You can use any combination of your favorite chocolate chips as long as they total 2 cups. Nut lovers can sprinkle an extra 1 cup chopped walnuts over filling