In medium bowl, beat butter, powdered sugar and 3 tablespoons of the milk with electric mixer on medium speed 1 to 2 minutes or until smooth and creamy. Beat in up to 1 tablespoon milk if necessary to make spreadable. Beat in food color until well mixed. Spread evenly over cooled bars. Sprinkle evenly with remaining cookies, slightly pressing into frosting. For bars, cut into 4 rows by 4 rows. Store covered in refrigerator up to 3 days.