Combine first 3 ingredients in a medium saucepan; add milk, stirring with a whisk until combined. Cook over medium heat 8 to 9 minutes or until thick, stirring constantly.
Place eggs in a small bowl; stir lightly with a whisk to combine. Gradually add one-fourth of hot milk mixture to eggs, stirring constantly. Add to remaining hot milk mixture in pan, stirring constantly. Bring mixture to a simmer over medium-low heat, stirring constantly. Cook 3 minutes. Remove from heat; stir in melted chocolate and lime rind.
Spoon mixture evenly into tart shells. Cover the surface of filling of each tart with plastic wrap, and chill 2 hours or until thoroughly chilled. Remove tarts from aluminum pans before serving, if desired. Top each tart with 1 tablespoon whipped topping and, if desired, a lime slice.