1Heat oven to 350º.
2In 1-quart saucepan, melt butter and chocolate over low heat, stirring constantly. Cool 5 minutes.
3In medium bowl, beat sugar, vanilla and eggs with electric mixer on high speed 5 minutes. Beat in chocolate mixture on low speed, scraping bowl occasionally. Beat in flour just until blended, scraping bowl occasionally. Stir in walnuts. Spread in frozen pie crust.
4Bake 37 to 42 minutes or until toothpick inserted halfway between center and edge of pan comes out almost clean. Cool in pan on wire rack, at least 2 hours. Cut into wedges. Serve with whipped cream, if desired.