Mix the graham cracker crumbs, sugar, and melted butter until it looks like wet sand, press firmly into the bottom and sides of a 9 inch pie dish, and freeze for 20 minutes.
Beat the cream cheese for 2 minutes until completely smooth. Getting rid of every lump now means a silky filling later.
Beat in the condensed milk, then add the key lime juice and lime zest and beat for 1 minute. The filling thickens noticeably as the lime juice reacts with the milk.
In a separate bowl, whip the cold cream to stiff peaks and fold it into the lime mixture in two additions.
Spread the filling into the chilled crust and refrigerate for at least 6 hours, or overnight, until set.
Whip the topping cream with the powdered sugar, swirl it over the pie, finish with lime zest, and serve cold.
