heat oven to 350°. Make pastry. Heat chocolate and water over low heat until melted. Add 2/3 cup sugar. Heat to boiling, stirring constantly. Remove from heat. Stir in margarine and 1 1/2 teaspoon vanilla. Chocolate mixture will be thick.
Beat remaining ingredients except egg and nuts in small mixer bowl on low speed until blended. Beat on medium speed, scraping bowl constantly, two minutes. Add egg. Beat, scraping bowl frequently, two minutes more. Pour into pastry lined pie plate. Stir chocolate mixture. Pour over mixture in pie plate. Sprinkle with nuts. Bake until wooden pick inserted in center comes out clean, 55 to 60 minutes. Serve with sweetened whipped cream if desired.