Whipped cream and walnuts for topping, optional * NOTE: the book did not specify what type of chocolate to use. Make your favorite single 20 cm pie shell. Cream butter, add sugar slowly, creaming well. Blend in melted and cooled chocolate and vanilla. Add eggs, one at a time, beating 5 minutes after each addition. With electric mixer use medium speed. Turn into cooled, baked pie shell. CHill 1 to 2 hours. Before serving top with whipped cream and walnuts, if desired.