In medium saucepan, stir together sugar and cream of tartar until well combined. Add eggs and water. Beat until well blended. Add the 4 squares of chocolate. Cook over low heat, stirring constantly, until chocolate is melted and mixture reaches at least 160 degrees. Remove from heat. Stir in vanilla. Set pan in bowl of ice water and stir vigorously until mixture is 80 to 90 degrees. Remove from ice water. Set aside. In small mixing bowl at medium speed, beat butter until light and fluffy, about 3 to 4 minutes. Add cooled egg mixture. Beat at high speed 4 to 5 minutes. Pour into pie shell. Chill before serving, at least 6 hours. Dollop with whipped cream and sprinkle with the slivered almonds and the shaved chocolate, if desired. Refrigerate any leftovers. Makes 6 to 8 servings.