Prepare and bake Piecrust in a 10-inch deep-dish pie plate. Cool completely on a wire rack.
To prepare filling, combine 1/2 cup sugar, cocoa, flour, and 1/4 teaspoon salt in a medium saucepan; stir with a whisk. Gradually stir in milk. Bring to a boil over medium heat, stirring constantly. Reduce heat; cook 2 minutes or until thick and bubbly, stirring constantly.
Remove from heat; add chopped chocolate, stirring until chocolate melts. Spoon chocolate mixture into a bowl; place bowl in a large ice-filled bowl for 10 minutes or until chocolate mixture comes to room temperature, stirring occasionally. Remove bowl from ice.
To prepare meringue, place egg whites and 1/4 teaspoon salt in a large bowl; beat with a mixer at high speed until soft peaks form. Combine 1 1/4 cups sugar and water in a saucepan; bring to a boil. Cook, without stirring, until candy thermometer registers 238°. Pour hot sugar syrup in a thin stream over egg whites, beating at high speed until stiff peaks form. Fold 2 cups meringue into chocolate mixture.
Spread chocolate mixture into prepared crust. Spread remaining meringue over chocolate mixture. Chill 8 hours; garnish with grated chocolate, if desired.