Combine 1 cup sugar, cornstarch, and salt in a heavy saucepan; stir well.
Combine milk and egg yolks; beat with a wire whisk 1 to 2 minutes or until frothy.
Gradually stir into sugar mixture, mixing well.
Cook over medium heat, stirring con-standy, until thickened and bubbly.
Remove from heat; add chocolate, butter, and vanilla, stirring until chocolate and butter melt Spoon into pastry shell; set aside.
Beat egg whites (at room temperature) and cream of tartar at high speed of an electric mixer 1 minute.
Gradually add 1/4 cup plus 2 tablespoons sugar, 1 tablespoon at a time, beating until stiff peaks form.
Spread meringue over filling, sealing to edge of pastry.
Bake at 350° for 10 to 12 minutes or until golden brown.