Combine cookie crumbs, coconut and melted butter. Press half of mixture onto bottom and up sides of 10 inch glass pie dish, building up sides to form an edge. Reserve remaining mixture for topping.
Combine 4 tablespoons butter, lime juice, cornmeal, salt and 3/4 cup sugar in large metal bowl set over pot of simmering water. Cook slowly, scraping sides of bowl and whisking occasionally, 15 minutes. Add the 6 egg yolks all at once and cook, whisking occasionally until thickened, about 15 minutes. Press plastic wrap onto surface of custard. Refrigerate until cool, whisking occasionally.
Preheat oven to 275 degrees F. Beat egg whites, cream of tartar and remaining 1/2 cup sugar until stiff and glossy. Gently fold half of beaten whites into lime custard. Then fold in remaining whites and lime zest.
Pour filling into pie shell and sprinkle with reserved topping, covering evenly. Bake 45 minutes, until pie has risen and coconut is golden brown. (The filling should be slightly cracked and firm when pressed in the center). Cool completely. Cover with plastic and refrigerate until well chilled, at least 6 hours or overnight.
c.1997, M.S. Milliken & S. Feniger, all rights reserved