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 35
To prepare meringue, combine 1 cup sugar and water in a saucepan; bring to a boil. Cook, without stirring, until candy thermometer registers 24
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.