Place 1 1/2 cups powdered sugar in 1-gallon resealable food-storage plastic bag; set aside. Place 12 cooled cookies in large bowl. Pour one-third of chocolate mixture over cookies in bowl. Using rubber spatula, toss cookies gently to coat. Place 3 cookies at a time in bag with powdered sugar; seal bag. Gently turn bag to coat cookies. Remove cookies from bag to cooling rack to set. Repeat with 12 more cookies and half of remaining chocolate mixture, rewarming if needed; repeat again until all cookies are coated, adding additional powdered sugar to bag as needed.