For meringue, in a medium mixing bowl, combine egg whites, granulated sugar, cream of tartar, and salt. Beat with an electric mixer on high speed for about 6 minutes or until stiff peaks form (tips stand straight).
Lightly spray a 9-inch pie plate with the nonstick cooking spray.
Fill a pastry bag fitted with a No. 3 or No. 4 star tip with meringue. Beginning in the center and working in a spiral outward, pipe meringue over bottom and up side of plate. (Or use a spoon to spread meringue into pie plate, building up side.)
Bake in the center of a 300 degree F oven for 1 hour or until lightly browned. Turn off oven. Let shell dry in the oven with door closed for 2 or more hours. Cover and store at room temperature.
For filling, in a large heavy saucepan, combine the 1/2 cup whipping cream, semisweet chocolate, and bittersweet chocolate. Cook and stir over low heat until the chocolates melt and the mixture is smooth. Remove from the heat. Cool slightly.
Slowly stir in the 4 cups whipping cream, blending well. Transfer mixture to a large mixing bowl. Add vanilla. Cover and chill in the refrigerator thoroughly for 4 hours or overnight.
Beat chocolate mixture with electric mixer on medium to high speed until soft peaks form (don't overbeat). Mound into meringue shell; flatten top with a spatula. Cover and store in refrigerator until serving time.
Just before serving, in a medium mixing bowl, beat the 1 cup whipping cream and powdered sugar with electric mixer on medium to high speed until stiff peaks form (tips stand straight). Using a pastry bag fitted with a star tip, pipe whipped cream onto pie or dollop over the top. Sprinkle with toasted walnuts. Makes 10 servings.