KEY LIME PIE

Preheat the oven to 175°C (350°F). In a food processor, process the biscuits until they resemble coarse crumbs. Put into a medium sized bowl and add as much of the melted butter as needed so that all the crumbs are coated.
Press the crumbs evenly over the bottom and up the sides of a 9-inch pie dish (I found the 9 inch pie dish to be a tad too small to hold all the mixture – I think you could have a slightly bigger dish). Bake the crust for about 15 minutes, or until firm. Let cool.
In a large bowl, beat the 3 egg yolks with the whole egg until thickened (about 3 minutes). Beat in the condensed milk and crème fraiche (or sour cream), then beat in the lime juice and zest.
In a stainless steel bowl, beat the egg whites until firm peaks form. Fold one-third of the beaten whites into the mixture, then fold in the remaining whites. Pour the mixture into the pie crust and bake for about 15-20 minutes (mine took closer to 25 minutes), or until barely set (the middle should still wobble a little). Transfer to a rack and let cool completely, then refrigerate overnight (it will continue to firm as it chills).
About an hour before serving, whip the cream until firm peaks form and spread over top of the pie and garnish with lime zest. Pop back into the refrigerator until ready to serve.