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