Preheat oven to 375 degrees.
Beat butter until creamy in small mixer bowl. Gradually add sugar, beating until light and fluffy. Add egg yolks and vanilla; beat until well blended. Stir in cocoa, flour and salt; blend well. (Batter will be very thick.) Stir in walnuts.
Beat egg whites in small bowl, until stiff, but not dry; fold into chocolate mixture. Spoon filling into unbaked pie crust.
Bake 30 to 35 minutes or until puffed across top. Cool completely. Garnish with whipped cream, if desired.