Blend the cookies or crush them with a rolling pin to a flour-like consistency.
Put the crushed cookies in a bowl and add the melted butter. Mix well.
Line a pie plate with the cookie mixture and place in the refrigerator while you make the filling.
Put the gelatin and cold water in a bowl and set aside.
Beat the cream to stiff peaks and set aside.
Mix together the condensed milk, lemon juice, and lemon zest.
Warm the gelatin-water mixture in the microwave 20-30 seconds. Add the warmed gelatin to the condensed milk and lemon mixture. Add the beaten cream. Fold the mixture together carefully and slowly.
Pour the mixture into the pie crust and refrigerate at least 6 hours. It's best to prepare the pie the day before serving.
For the topping: mix lemon juice, sugar, and corn starch in a heavy saucepan. Place over low heat.
Warm the gelatin-water mix in the microwave 20-30 seconds. Allow to cool, then add to the topping mixture. Mix well.
Allow the topping to cool and then pour it gently over the pie filling. Allow to rest until set.