For Coconut Custard Filling: Whisk the egg yolks in a bowl, set aside. Stir cornstarch, sugar and salt together in a medium saucepan. Whisk in the milk carefully to avoid lumps. Add egg yolks. Cook, stirring constantly, over medium heat until custard starts to thicken. Remove from heat. Add butter and stir until melted. Set aside and wait about 8 minutes stirring to cool the mixture. Add vanilla and coconut then pour into the baked pie shell.
For Topping: Beat the egg whites until soft peaks form, then add sugar, 1 tablespoon at a time, beating after each. Whip until peaks become creamy. Spread whipped mixture on the pie and then sprinkle coconut on top. Preheat oven to 500 degrees F and brown the topping. Yields: 8 to 10 slices.