Combine flour, cornstarch, egg, egg yolk, and 1/2 cup milk in a small mixing bowl; stir until smooth. Set aside.
Combine remaining milk, 1/2 cup sugar, and salt in a medium saucepan. Cook over medium heat, stirring constantly, until mixture begins to boil. Slowly add reserved flour mixture, stirring constantly. Continue cooking, stirring constantly, until mixture is thickened and bubbly. Remove from heat; add butter and vanilla. Stir until butter melts. Set aside.
Beat egg whites (at room temperature) until foamy. Gradually add remaining 1/4 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently fold egg whites into custard, blending well. Spoon mixture into pastry shell. Sprinkle top of pie with 1 1/2 cups coconut. Toast remaining coconut and sprinkle over top. Chill thoroughly before serving.