Mix base cake and butter until well blended; press firmly onto bottom of 1 full hotel pan (or 1 half hotel pan for trial recipe). Sprinkle with OREO Cookie Pieces, coconut and walnuts. Drizzle condensed milk evenly over ingredients in pan.
Bake at 350°F for 20 to 25 minutes or until golden brown. Cool completely.
Cut into 48 squares (or 24 squares for trial recipe). Store covered in refrigerator.