Combine 1/2 cup of the sugar, cornstarch and salt in a saucepan. Add the milk and cook over medium heat, stirring constantly, until mixture thickens slightly. Remove from heat and set aside
Beat the egg yolks with an electric mixer at high speed until thick and lemon colored. Stir about one-fourth of the custard mixture into the yolks. Blend well. Then stir the blended mixture back into the hot custard. Cook over medium heat for 2 to 3 minutes, stirring constantly. When mixture thickens to custard consistency, remove from heat. Stir in the butter, 3/4 cup coconut and 1 teaspoon of the vanilla until butter has melted. Pour mixture into the baked pie shell.
Beat the egg whites at high speed with an electric mixer just until foamy. Add the cream of tartar and vanilla, and gradually add the remaining 1/4 cup sugar. Continue beating until stiff peaks form and sugar has dissolved, about 2 minutes.
Spread meringue over the pie filling, sealing to edge of pastry. Sprinkle with remaining 1/4 cup coconut. Bake at 425°F for 5 minutes or until meringue is lightly browned.