MIX 1 cup sugar and cornstarch in medium saucepan. Stir in chocolate. Gradually stir in milk. Bring to full rolling boil on medium heat, stirring constantly; cook and stir 1 min. Remove from heat.
BEAT egg yolks lightly in small bowl. Remove about 1/4 cup milk mixture; mix with yolks. Gradually add to milk mixture in saucepan, mixing well after each addition. Return to boil, stirring constantly; cook and stir 1 min. Remove from heat.
ADD butter and vanilla; stir until butter is completely melted and mixture is well blended. Pour into crust; cover with plastic wrap, pressing wrap gently onto surface of pie. Refrigerate 3 hours.
BEAT cream and remaining sugar with mixer on high speed until mixture forms soft peaks; spread onto pie.