Mix 1 1/2 cups sugar, cocoa, flour, and salt in a large saucepan. Stir in 1 egg; beat 3 egg yolks and add to cocoa mixture, mixing well. Pour boiling water over cocoa mixture, stirring constantly. Cook and stir over medium heat for 5 to 10 minutes or until mixture thickens. Remove from heat and add vanilla. Pour into pastry shell.
Beat 3 egg whites until foamy in a small mixing bowl. Add remaining sugar gradually, beating until mixture forms stiff peaks. Spoon on top of pie; bake at 350° for 10 minutes or until meringue is browned. Sprinkle top with shaved chocolate.
Note: Meringue may be omitted and whipped cream used as topping instead.