1 pie shell, baked and cooled
1 cup sugar
3 tablespoons cornstarch
1/4 teaspoon salt
2 tablespoons cocoa powder
3 eggs, separated
2 cups milk
1 tablespoon vanilla extract
2 to 3 tablespoons butter
1/4 teaspoon cream of tartar

Heat oven to 350 F. In a medium microwave-safe bowl, stir together sugar, cornstarch, salt and cocoa. Add egg yolks and milk. Beat to combine. Cook in microwave around 3 minutes or until thick, stirring after about 1 1/2 minutes to prevent scorching. Add vanilla and butter. Mix until smooth. Pour into crust. 

With an electric mixer, beat the 3 egg whites and cream of tartar until stiff peaks form. Cover pie with meringue. Bake until light brown.

Recipe note: Omit cocoa powder and add 1 cup sweetened coconut flakes to make this a coconut pie. Sprinkle meringue with coconut before baking.