Stir boiling water into dry gelatin in large bowl at least 2 minutes until completely dissolved. Stir in cold water. Pour 1 & 1/2 cups of the gelatin into 9-inch round cake pan sprayed with cooking spray. Refrigerate 2 hours until set but not firm (gelatin sticks to finger when touched and should mound).
Add milk to remaining gelatin; stir until well blended. Add dry pudding mix. Beat with wire whisk 2 minutes or until well blended. Gently stir in 2 cups of the whipped topping and the mandarin oranges. Spoon over gelatin layer in pan. Arrange cookies on pudding mixture to form crust (pan will be full). Cover with plastic wrap.
Refrigerate 4 hours or until firm. To unmold, run small metal spatula around edge of pan. Dip pan in warm water, just to rim, for about 10 seconds. Place serving plate on top of pan. Invert; holding pan and plate together, shake gently to loosen. Carefully remove pan. Garnish with remaining whipped topping. Store leftover dessert in refrigerator.