POSITION oven rack in lower third of oven. Preheat oven to 375°F. Prepare pie crust as directed on package, using 9-inch pie plate; set aside. Microwave chocolate in large microwaveable bowl on HIGH 2 min. Stir until chocolate is completely melted; set aside.
BEAT butter and sugar in large bowl with electric mixer on medium speed until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Blend in melted chocolate and vanilla. Add flour; mix well. Stir in chopped walnuts. Pour into crust. Arrange walnut halves over filling.
BAKE 25 min. or until center of filling is set. Cool completely. Refrigerate at least 1 hour before serving.