1. Prepare pastry shell; cool.
2. Combine chocolate and 2 cups milk in medium saucepan; cook over medium heat, stirring constantly, just until mixture boils. Remove from heat.
3. Stir together sugar, flour, cornstarch and salt in medium bowl. Blend remaining 1 cup milk into egg yolks; stir into sugar mixture. Gradually add to chocolate mixture. Cook over medium heat, stirring constantly with wire whisk, until mixture boils; boil and stir 1 minute. Remove from heat; stir in butter and vanilla.
4. Pour into prepared pastry shell; press plastic wrap directly onto surface. Cool; refrigerate until well chilled. Top with whipped cream, if desired. 8 to 10 servings.