Preheat oven to 375 deg F. Crush cookies into fine crumbs, and blend with sugar and melted butter. Press mixture into bottom and side of a 9-inch pie plate. Bake 6 minutes then cool on wire rack. Set aside.
In a bowl, mix cornstarch with a cup of the milk. Add eggs and vanilla extract, and beat well. Set aside.
In a saucepan over low heat, combine the remaining 2 cups milk and sugar, and bring to a boil. Add egg mixture and stir until thickened.
Remove from heat. When mixture is cool, pour 1/3 into a bowl and the remaining 2/3 into another.
Melt chocolate in a double boiler over barely simmering water. Stir melted chocolate into 1/3
of the filling and blend well. Pour into pie crust and refrigerate.
In a saucepan over low heat , dissolve gelatin in water combine with remaining 2/3 of the filling mixture. Blend in cognac.
In another bowl, beat egg whites until stiff peaks form. With a rubber spatula, fold egg whites gently into filing mixture. pour over chocolate layer in pie plate. Refrigerate for 12 hours.