To prepare filling, combine 1/2 cup milk, 2/3 cup sugar, and next 4 ingredients (2/3 cup sugar through egg) in a large bowl, stirring with a whisk.
Heat 1 1/2 cups milk in a heavy saucepan over medium-high heat to 180° or until tiny bubbles form around edge (do not boil). Remove from heat. Gradually add hot milk to sugar mixture; stir constantly with a whisk. Return milk mixture to pan. Add chopped chocolate; cook over medium heat until thick and bubbly (about 5 minutes); stir constantly. Reduce heat to low; cook 2 minutes, stirring constantly. Remove from heat; stir in vanilla. Pour into crust; cover surface of filling with plastic wrap. Chill 3 hours or until cold.
Preheat oven to 350°.
To prepare meringue, combine 1 cup sugar and water in a saucepan; bring to a boil. Cook, without stirring, until candy thermometer registers 240°. Beat egg whites and cream of tartar at high speed of a mixer until foamy. Pour hot sugar syrup in a thin stream over egg white mixture; beat at high speed until stiff peaks form. Remove wrap from filling. Spread meringue evenly over filling; seal to edge of crust.
Bake at 350° for 15 minutes or until lightly browned; cool on a wire rack. Chill until set.