PREHEAT oven to 350°F. Place caramels and 1/3 cup of the milk in heavy saucepan; cook on low heat until caramels are completely melted, stirring frequently. Remove from heat; set aside. Mix remaining milk, dry cake mix and butter in large bowl. Press half of the cake mixture firmly onto bottom of 13x9-inch baking pan to form crust. Bake 10 minutes.

SPRINKLE 1 cup of the walnuts and 1/2 cup of the chocolate chunks over crust. Top with caramel mixture; carefully spread to edges of pan. Top with teaspoonfuls of the remaining cake mixture; press gently into caramel mixture with back of spoon. Sprinkle with the remaining 1/2 cup each walnuts and chocolate chunks.

BAKE an additional 18 minutes or until lightly browned. Cool completely. Cut into 32 bars to serve.